What is the punctuation sign to show omission of a letter or letters in word?

The punctuation sign to show the omission of a letter or letters in a word is an apostrophe (').

Here are some examples:

* don't (for "do not")

* can't (for "cannot")

* I'm (for "I am")

* it's (for "it is")

* o'clock (for "of the clock")

The apostrophe is placed where the letter(s) are omitted.