Every criminal act is illegal or unlawful, but illegal or unlawful acts may not be criminal. Offenses against public law are criminal; offenses against private rights are merely illegal or unlawful.

J. C. Fernald, English Synonyms and Antonyms (1914)

Example sentences

criminal Fadil was engaged in a lot of criminal activities.
illegal Do you think that handguns should be made illegal?
illicit Sami was too involved in an illicit relationship.
illegitimate The king had numerous illegitimate children with her.
irregular Rainfall in Algeria has become rare and irregular.
improper It is improper to impose sanctions upon the union.

Popularity over time

Word	1850 → 2019	Trend
unlawful		falling
criminal		steady
illegal		steady
illicit		rising
illegitimate		steady
irregular		falling

Use in printed books, per decade from 1850 to 2019.

By length

7 letters: illegal³, illicit³, lawless²
8 letters: criminal³, improper³, wrongful², outlawed²
9 letters: irregular⁴, unlawlike
10 letters: contraband³, unlicensed³, unlicenced
12 letters: illegitimate⁵
14 letters: unconventional⁵

The small number after each word is its number of syllables (illegal³ = 3 syllables).