Anything is unnatural when it departs in any way from its simple or normal state; it is factitious when it is wrought out or wrought up by labor and effort, as, a factitious excitement.

Webster's Revised Unabridged Dictionary (1913)

Example sentences

strange Press the red button if something strange happens.
abnormal It's abnormal to have the heart on the right side.
irregular Rainfall in Algeria has become rare and irregular.
aberrant Tom's aberrant thinking persists.
artificial You can drink the juice since it's not artificial.
stilted She expresses herself in a rather stilted manner.

Popularity over time

Word	1850 → 2019	Trend
unnatural		rising
strange		rising
abnormal		steady
irregular		falling
aberrant		rising
artificial		falling

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

By length

4 letters: hard¹
5 letters: stiff¹, kinky²
6 letters: forced¹
7 letters: strange¹, unusual⁴, violent³, erratic³, deviant³, labored², stilted², feigned¹
8 letters: affected³, uncommon³, abnormal³, peculiar³, singular³, strained¹, mannered², aberrant³, laboured, freakish
9 letters: irregular⁴, eccentric³, monstrous², grotesque², perverted³, contrived², anomalous⁴, subnormal
10 letters: artificial⁴, outlandish³, factitious³
11 letters: exceptional⁴, constrained², antinatural
13 letters: preternatural, contranatural
14 letters: unconventional⁵

The small number after each word is its number of syllables (kinky² = 2 syllables).