-que

This word never stands on its own. It is an enclitic: it attaches to the END of the second item and joins it to what came before, so "the senate and the people" is senatus populusque, not senatus que populus. Our frequency count lists it as a separate word because the counting tool strips it off before matching, which is a useful fiction rather than a fact about the language.

The difference from et is one of grip. Enclitic -que binds tightly and suits pairs that already belong together, natural or formulaic ones: terra marique, noctes diesque, Senatus Populusque Romanus. Use et for a loose list and atque or ac when you want the second item to land with weight. Doubled, -que ... -que gives "both ... and", mostly in verse. It is also fossilised inside a whole shelf of ordinary words: itaque, namque, quisque, uterque, plerique, and atque itself, which is ad plus -que.