The shortening of two words into one, where an apostrophe is used in place of the omitted letter or letters (I’m, aren’t); or the abbreviation of a word to its first and last letters (Mr, St for “mister”, “street”, “saint”).

Omission of a sound between two words (usually a vowel and the end of one word or the beginning of the next).
Omission of certain sounds or syllables in speech. In writing, elision may be shown by contraction, but it is not practical or desirable to show all the elisions that occur in rapid or informal speech.