A negative number is a number which is less than zero.

Negative numbers
are directed numbers used to measure in the opposite sense to positive
numbers. They are written with a minus sign. For example, -3, -1.5,
-7/4.

Sometimes upper height minus signs are used to distinguish the sign
from subtraction: -3, -1.5, -7/4.

Temperatures below zero are described using negative numbers.
For e.g., a temperature of 4 degrees below zero is called
negative 4, or more usually minus 4, and is written –4.