Aspiration

In phonetics, the term aspiration means a strong burst of air accompanying the release of some stops. English voiceless stops are aspirated when they begin a stressed syllable (as in pen, ten, Ken), but this aspiration is not distinctive (they also have unaspirated variants in other positions). In many languages [t] and aspirated [t^h] (not to be confused with the English sound spelt th as in thin) are different phonemes.

For example, put your hand in front of your mouth and say 'key' and then 'ski'. Key is aspirated, and ski is unaspirated.