complex
Noun pluralcomplexes
Pluralcomplexes
Formscomplexes, complexing, complexed
- 1Difficult to understand or deal with because it is not simple or straightforward; complicated.
The negotiations grew increasingly complex as more parties joined the table.
She gave a complex answer that took several minutes to unpack.
UsageOften highly complex or increasingly complex. Complicated is the more everyday word for the same idea.
Similarcomplicated, intricate, difficult, involved, hard
Oppositesimple, easy, basic, straightforward
Note
Here the many parts become a burden on the mind. Where the "many parts" meaning describes structure, this one describes the effort of following it. William Whewell, writing in 1837, pairs it with difficult when describing how the heavens' motions are calculated, and blames nature rather than the astronomer.
Checked 2 Nov 2026 · 3 facts
- William Whewell's cited passage appeared in 1837. archive.org
- Whewell pairs complex with difficult when discussing calculations of celestial motions. archive.org
- Whewell directs dissatisfaction at nature rather than the astronomer. archive.org
- 2A group of buildings that serve a shared purpose, such as a university or military base.
The hospital complex covers three city blocks and employs thousands of staff.
Visitors must sign in at the gate before entering the research complex.
UsageUsually after a word for its purpose: a sports complex, a housing complex, a hospital complex.
- 3Made up of multiple connected parts rather than a single whole; composite.
Proteins are complex molecules built from long chains of amino acids.
The orchestra produced a complex sound from dozens of instruments at once.
UsageCompares as more complex, most complex (not
incorrect: complexer).Similarcomposite, compound, intricate, complicated, elaborate
Oppositesimple, basic, elementary, uncomplicated
Note
John Locke's Essay of 1689 helped make it a term of philosophy, calling ideas built from several simple ones complex: beauty, gratitude, an army, the universe.
Checked 2 Nov 2026 · 2 facts
- John Locke's Essay first appeared in 1689. en.wikipedia.org
- Locke calls ideas assembled from simple ideas complex and gives beauty, gratitude, an army and the universe as examples. en.wikisource.org
- 4A group of emotionally charged, often unconscious ideas or an obsession about a subject that shapes a person's feelings and behavior.
Years of criticism left him with a deep inferiority complex.
Her guilt complex made it impossible to enjoy any of her successes.
UsageUsually in compounds like an inferiority complex or a guilt complex, or have a complex about something.
Note
Psychoanalysis gave the word its technical meaning, a cluster of repressed ideas. In everyday use that background falls away, and only the fixation remains.
Checked 2 Nov 2026 · 2 facts
- Psychoanalysis established this technical sense of complex. gutenberg.org
- In psychoanalysis, a complex is a cluster of associated ideas, discussed in connection with repression. psychclassics.yorku.ca
- 5A chemical unit formed when two or more atoms, ions, or molecules bind together, such as a metal ion bonded to surrounding ligands in a coordination compound, or protein subunits joined in a protein complex.
The metal ion forms a stable complex with six ammonia ligands.
Hemoglobin is a protein complex that carries oxygen through the bloodstream.
Note
Chemistry's complex is defined by association: atoms, ions, or molecules held together as one unit, often but not always around a central metal atom. The term spans two fields, with coordination complexes in inorganic chemistry and protein complexes in biochemistry. The noun has its own verb, to complex, for the act of forming one.
- 6A group of related or interconnected things or systems considered or functioning together as a whole.
Historians study the complex of customs that shaped medieval village life.
The report describes a complex of factors driving the recent price increase.
UsageUsually a complex of things: a complex of factors.
Similarnetwork, system, web, cluster, combination
Note
The general sense from which several specialist ones branch: weather's storm complex, firefighting's fire complex and taxonomy's species complex all name assemblages of this kind. Robert South's Twelve Sermons speaks of "the whole complex of all the blessings and privileges" of the gospel. That abstract, gathered-up use survives today in phrases like a complex of factors.
Checked 31 Oct 2026 · 2 facts
- Robert South wrote the work referred to as Twelve Sermons, containing the quoted passage. en.wiktionary.org
- South uses the exact quoted wording about the blessings and privileges offered by the gospel. ccel.org
Less common meanings
- 7In mathematics, of a number, or a function, matrix, or polynomial: being or involving numbers with a real and an imaginary part.
The polynomial's complex roots come in conjugate pairs.
Engineers use complex impedance to model alternating-current circuits.
Note
The foundational mathematical sense, said of numbers: complex number, function of a complex variable. Here the word contrasts with real, not with simple. A complex number is no harder than any other; it simply has two parts. Many other mathematical uses lean on this one, from complex-valued functions to the bare noun a complex for the number itself.
Checked 31 Oct 2026 · 2 facts
- A complex number has real and imaginary parts. en.wikipedia.org
- Some mathematical uses are independent: an abstract simplicial complex is a collection of sets, not a construction requiring complex numbers. people.math.harvard.edu
- 8A group of closely related species that are often difficult to tell apart using traditional physical features.
DNA testing showed that the lizards on the island form a complex of several look-alike species.
Taxonomists revised the complex after DNA analysis revealed hidden diversity.
Note
A taxonomist's word for a knot of look-alike species, in full a species complex, also called a species group or species aggregate. It keeps company with cryptic, meaning species hidden inside what was named as one, and with unresolved, because a complex is a problem still being worked out. Mosè Manni and colleagues in 2015 call the nominal species A. fraterculus "an unresolved complex of cryptic species."
Checked 31 Oct 2026 · 2 facts
- Mosè Manni and colleagues published the cited statement in 2015. ri.conicet.gov.ar
- They describe the nominal species A. fraterculus using the exact phrase "an unresolved complex of cryptic species." ri.conicet.gov.ar
- 9
- 10A number having both a real part and an imaginary part, written in the form a + bi.
The interesting aspect here is that U₃ is irreducible, even though all irreps over the complexes are one-dimensional because ℤ₄ is abelian.
1996, Barry Simon, Representations of Finite and Compact Groups - 11A group of wildfires burning near one another, often managed together as a single event.
- 12An organized cluster of thunderstorms acting together as a single weather system.
- 13To make something more complicated, adding difficulty or extra detail to it.
- 14A word built from several meaningful parts, such as a root combined with one or more affixes.
- 15Of a curve or polygon, crossing or intersecting itself rather than staying simple.
Showing meanings only.
Practice
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Origin
From French complexe, from Latin complexus, the past participle of complector ("I entwine, encircle, infold"), formed from com- ("together") and plectere ("to weave, braid").
Checked 31 Oct 2026 · 4 facts
- English complex comes from French complexe, from Latin complexus. en.wiktionary.org
- Complexus is the past participle of complector, meaning to entwine, encircle or infold. en.wiktionary.org
- The underlying verb combines com-, meaning together, with the verb meaning weave or braid. en.wiktionary.org
- Plectere is the infinitive of plecto and means to weave or braid. en.wiktionary.org