Final answer:
The process of removing an unstressed syllable or letter from a word or phrase and replacing it with an apostrophe is known as forming a contraction.
Step-by-step explanation:
The removal of an unstressed syllable or letter from a word or phrase, which is replaced with an apostrophe, is called a contraction. A commonly encountered example of a contraction is the shortening of the word cannot to can't, where the omitted letter o is replaced by an apostrophe. Another instance is the word over, which can be contracted to o'er in poetry to fit the metrical pattern of the verse.
Contractions serve to make speech and writing more informal and conversational, and are widely used in English language. It's important to recognize that apostrophes are not used for plural nouns, singular verbs, or personal pronouns, but they are used to show possession or to form contractions.