Perpetual denotes, in its absolute sense, what literally never ceases or comes to an end; as, perpetual motion. In its qualified sense, it is used hyperbolically, and denotes that which rarely ceases; as, perpetual disturbance; perpetual noise; perpetual intermeddling.

Webster's Revised Unabridged Dictionary (1913)

Example sentences

permanent It is difficult to find a well paid permanent job.
eternal The eternal flame of love burned only half a year.
unending I'm up to here with your unending, insufferable nagging!
constant I rediscovered the importance of constant efforts.
ceaseless Her ceaseless caterwauling alarmed every dog within a 500-foot radius.

Popularity over time

Word	1850 → 2019	Trend
perpetual		steady
permanent		falling
eternal		rising
unending		rising
constant		falling

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

By length

5 letters: fixed¹
6 letters: stable², eonian
7 letters: endless², eternal³, undying³, ageless², aeonian
8 letters: constant², infinite³, immortal³, enduring³, timeless², habitual⁴, unending³
9 letters: permanent³, perennial⁴, evergreen³, continual⁴, incessant³, immutable⁴, ceaseless², unfailing³, unceasing³
10 letters: continuous⁴, persistent³, relentless³
11 letters: everlasting⁴, unremitting⁴, sempiternal
12 letters: interminable⁵, unchangeable
13 letters: uninterrupted⁵, unintermitted

The small number after each word is its number of syllables (stable² = 2 syllables).

In other languages

Français: perpétuel
Italiano: perpetuo
Català: perpetu