We speak of a thing as accidental when it falls out as by chance, and not in the regular course of things; as, an accidental meeting, an accidental advantage, etc.

Webster's Revised Unabridged Dictionary (1913)

Example sentences

incidental His criticism only referred to incidental matters.
unintended This had some unintended consequences for Algeria.
unintentional The company claims that the delay was unintentional.
external The vagina meets the external organs at the vulva.
adventitious A rather adventitious source informed me of his past doings.

Popularity over time

Word	1850 → 2019	Trend
accidental		falling
incidental		falling
unintended		rising
unintentional		rising
inadvertent		steady
external		steady

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

By length

6 letters: chance¹, random², casual³
8 letters: accident³, external³
9 letters: unplanned², unwitting³, penumbral, causeless
10 letters: unexpected⁴, occasional⁴, contingent³, unintended⁴, incidental⁴, unforeseen³, extraneous⁴, fortuitous⁴
11 letters: inadvertent⁴, inessential, unessential
12 letters: adventitious
13 letters: unintentional⁵, serendipitous⁵
14 letters: circumstantial⁴

The small number after each word is its number of syllables (random² = 2 syllables).

In other languages

Français: accidentel
Italiano: accidentale
Español: accidental
Català: accidental