When the final section or syllable of a word is cut off, it's called an apocope. The word "photo" is an apocope of "photograph."
While some apocopes appear in speech simply due to the way a person pronounces a word — saying mos instead of most, for example — most of them function more like nicknames for longer words. The now-common word zoo began as an apocope for zoological, and nearly everyone knows what you mean if you say obit instead of obituary. The word is rooted in the Greek apokoptein, "cutting off," from apo-, "away from," and koptein, "to cut."
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