open book

Short definition from Wiktionary

Noun

1. 1
   Something whose salient aspects are obvious or easily interpreted.
2. 2
   A person who through naivete responds candidly to questions or openly displays their emotions or intentions.
3. 3
   An open book decomposition.
4. 4
   An inside angle in the rock.

Adjective

1. 1
   Designating exams in which specified external resources, potentially including textbooks, glossaries, study notes, calculators, etc. may be used.