Minimal Pairs:

In phonetics, two words that differ in only one sound (a phoneme), such as hit and hid.

·      A minimal pair is a pair of words that differ in a single phoneme. Minimal pairs are often used to show that two sounds contrast in a language. For example, we can demonstrate that [s] and [z] contrast in English by adducing minimal pairs such as sip and zip, or bus and buzz. Since the only difference in these words is the [s] vs. [z], we conclude that they belong to distinct phonemes. However, a similar test would show that [a:j] and [Aj] are distinct phonemes in English, since writer and rider appear to be minimal pairs distinguished in their second elements, not their fourth."

(James Alasdair McGilvray, The Cambridge Companion to Chomsky. Cambridge University Press, 2005)