et al.
Also called: et alia, and others
Latin for “and others” — an abbreviation that stands in for the co-authors you are not naming in an in-text citation.
“et al.” abbreviates et alia. The period belongs on “al.” and not on “et”, because “et” is a whole word and “al.” is the abbreviation. It is never italicised in APA, MLA, or Chicago.
When it kicks in depends on the style. APA 7 uses et al. from the very first citation of any work with three or more authors. MLA 9 uses it for three or more. Chicago author–date lists up to three authors and switches at four. Vancouver and other numeric styles usually list six authors in the reference before truncating, but the in-text marker is just a number, so et al. never appears there.
In the reference list the rules are different again and more generous: APA 7 lists up to 20 authors before it elides, so a paper that reads “(Rivera et al., 2021)” in the text may well name eight people in the references.
APA 7, four authors
First and all later citations: (Rivera et al., 2021) — not (Rivera, Chen, Okafor, & Larsen, 2021).