Phonemic contrast is the property that allows the discrimination of distinctive speech elements of a language and the accompanying ability of a listener to distinguish meaning.
The term "phoneme" itself is defined in terms of contrast: The phoneme can be described as "The smallest contrastive linguistic unit which may bring about a change of meaning". In this context, a pair of close phonemes may also be called a phonemic contrast, to distinguish them from being allophones.
A phonemic feature is called contrastive, if it ensures a phonetic contrast for two otherwise similar phonemes. For example, vowel length may be contrastive in some languages and allophonic in others.
Even within a single language, a feature may or may not ensure contrast. For example, Thai has several sets of stop consonants that differ in terms of voicing (whether or not the vocal cords vibrate) and aspiration (whether a puff of air is released). Yet the language has no voiced velar consonant (/ɡ/). This peculiarity is called a phonological gap or a "hole in the pattern".
The phonemic contrast of a word may be formally defined in terms of the number of phonemically similar items in the lexicon of a particular language (dialect, variety, etc.), which is a measure of the ease of the recognizability of a word.
An interlanguage phonemic contrast (diaphonemic contrast) is the contrast required to differentiate between two cognate forms coming from two compared languages.
Some speech phenomena may lead to the neutralization of phonemic contrasts. For example, due to final-obstruent devoicing, Russian бес ('demon', phonemically /bʲes/) and без ('without', phonemically /bʲez/) are pronounced identically in isolation as [bʲɛs].