Definition
One
One is the number 1: a single unit and the first counting number.
What it means
What counts as one depends on the unit: one apple, one metre, or one whole shape. Once the unit is named, one means a single copy of it. In multiplication, × 1 keeps the other number the same.
Remember it
One means a single named unit.
Example
There are 6 crayons in each box and exactly 1 box. The total is 6 × 1 = 6, showing that multiplying by one does not change the number.
Watch for
One object is not always one whole: one slice may be only part of a pizza.
Common mix-up
The number one is the idea; the numeral 1 is a symbol used to write it.
Exact meaning
One is the number 1. It represents a single unit, is the first counting number, and is the multiplicative identity: multiplying any number by 1 leaves that number unchanged.