What is the meaning of 5?

The cardinal number five.

A digit in the decimal system of numbering, as well as octal, and hexadecimal.

the pitch of a tone, either high or low depending on local convention; '5' is most commonly the highest or lowest pitch (but see 6)

tone number 5, typically identified with dark departing yin qu (陰去)

Clipping of 540. (540° spin)

alternative form of (m4, not)

Source: wiktionary.org