Impact factor
Also called: journal impact factor, JIF
A journal-level metric: the average number of citations in one year to articles the journal published in the two preceding years.
The impact factor is computed by Clarivate for journals in Web of Science. Being an average over a skewed distribution, it is dominated by a handful of heavily cited papers, so it predicts very little about any individual article in the journal.
It also varies enormously by field — a strong mathematics journal has a lower impact factor than a middling cell-biology one — and it is gameable through review articles and editorial self-citation. Declarations such as DORA specifically discourage using it to assess researchers.
Use it, if at all, as one weak signal about a journal. It is not a measure of a paper and not a measure of a person.