'Criterion' is Greek for 'means of judging' — from 'krinein' (to sift), kin to 'crisis' and 'critic.'
A standard, rule, or principle by which something is judged or decided.
From Greek 'kritḗrion' (a means of judging, a standard, a test), from 'kritḗs' (a judge, an arbiter), from 'krínein' (to separate, to sift, to distinguish, to decide, to judge). The PIE root is *krey- (to sift, to separate, to distinguish) — an agricultural metaphor: grain is separated from chaff by sifting, just as a judge separates truth from falsehood, right from wrong. This root is one of the most conceptually rich in Indo-European. Greek reflexes include: 'krísis' (a decisive moment, the turning
The words 'criterion,' 'crisis,' 'critic,' and 'crime' all descend from the same PIE root *krey- (to sift, to separate). A crisis is a moment of separation (decision point), a critic is one who separates good from bad, a crime is what is separated out (judged) as wrong, and a criterion is the standard by which the separation is made.