Ellipsis refers to

The correct answer is 'omission of words'.

Key Points

Ellipsis is a grammatical term and, as such, is used in two main ways.

• It appears as a sequence of dots, usually three (…), which indicates the parts of a word or sentence that have been omitted. These are called ellipsis points. 
• It can also be used for more stylistic. This is when a word or phrase is left out, or omitted, from a sentence. The words omitted may be necessary to make a sentence syntactically correct but they are not necessary for a reader to fully understand the sentence’s meaning.

Thus, we can conclude that Ellipsis refers to the omission of words.