The linguistic term for the loss of a syllable in spoken word is syncope, but I simply refer to it as dropped syllables.
Other examples of dropped syllables are the words every, favorite, and different.
ev-er-y versus ev-ry
fav-o-rite versus fav-rite
dif-fer-ent versus diff-rent
Dropping syllables occurs mostly on high-frequency words, and dictionaries are pretty good about showing both options when two choices of pronunciation are available.
The syllable that can be dropped, not surprisingly, follows a pattern. The syllables before or after a stressed syllable in a word are often unstressed. (This is opposed to a secondary stress that can occur two syllables apart from a stressed syllable.) Only the vowel sounds of unstressed syllables can get dropped, and usually the original word needed to have at least three syllables to begin with. I mentioned four words above, which I'll repeat now.
Here are some more examples. (I'm only going to pronounce these the less formal way, with the dropped syllable):
vegetable /ˈvedʒ.tə.bl ̩/
mystery /ˈmɪs.t r.i/
The most common 2-syllable word can be reduced to a single syllable: s'pose (for suppose), as in "I s'pose I can help you tomorrow."
Also, like most informal options of pronouncing English, they may go away is the word is emphasized in a sentence. For instance, the word every. In normal speech, it drops to 2 syllables, every. However, if I were emphasizing that word, it may go back to the more proper 3 syllables, every. For example, in the sentence:
You don't need to practice every day, but you should try to most days.
I stressed the word every, and it was said with three syllables, as ev-e-ry.
Now, I do need to say, North Americans and British do this differently. So if you are more exposed to British English, you will not notice this to the same extent.