apropos

Short definition from Wiktionary

Adjective

1. 1
   Of an appropriate or pertinent nature.
2. 2
   By the way, incidental.

Preposition

1. 1
   Regarding, concerning, in regard to, on the subject of

Adverb

1. 1
   By the way.
2. 2
   Timely; at a good time.
3. 3
   To the purpose; appropriately.

Noun

1. 1
   Fittingness, pertinence.