What separates the units and the tenths what do you call it?

The separator between the units and tenths place in a decimal number is called the decimal point. It distinguishes the whole number part from the fractional part, with the digits to the left representing units and those to the right representing tenths and smaller fractions. For example, in the number 5.3, the decimal point separates the 5 (units) from the 3 (tenths).