noun

The English alphabet: ABC are the first three letters of the English alphabet. It is used to refer to the alphabet itself, as in "The alphabet starts with ABC."
Basics, introduction: The ABCs of something means "the basics of something."

Example

1. The teacher asked the students to write the abc on the board.
2. Learning the abc is the first step in reading and writing.
3. She sang the abc song to her little brother.