Impending, also from the Latin, with the sense of hanging over, is closely akin to imminent, but somewhat less emphatic. Imminent is more immediate, impending more remote, threatening more contingent.

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

Example sentences

upcoming Aren't you going to vote in the upcoming election?
pending A similar measure is pending again this year.
forthcoming Tom wasn't exactly forthcoming with an answer.
imminent We think that Tom might be in imminent danger.

Popularity over time

Word	1850 → 2019	Trend
impending		steady
upcoming		rising
pending		falling
forthcoming		steady
imminent		steady

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

By length

4 letters: near¹
5 letters: close¹
7 letters: instant², pending²
8 letters: upcoming³, imminent³
9 letters: incumbent³, proximate³, impendent
11 letters: threatening³, forthcoming³

The small number after each word is its number of syllables (instant² = 2 syllables).