Populace - populous
From Hull AWE
The two words populace and populous, which sound identical in the mouths of most native English speakers, are different, though closely related, in meaning. (They are virtually homophones; both stress the first syllable, which has the vowel of 'got', IPA: /ɒ/) The difference is largely due to their different word classes.
- Populace is a noun. It is a looser, more general, and altogether more impressionistic word than 'population' - but it means much the same. Unless you have good reason to use populace, prefer population in academic writing.
- Populous is an adjective. It means 'with many people'. "London is a populous city". This is a word you'd be best advised to avoid, in most formal writing. Use, as a substitute, the less stuffy, simpler words such as 'crowded' or 'busy', unless you mean to use 'populous' specifically as a quasi-technical term in subjects like Geography.
(For a related etymological point, you may like to see popular.)