A number (either whole number or not), for example 6 or -7.2
A number may have an attached unit (such as years, or metres per second), written in curly brackets afterwards. To add or subtract two numbers, they must have the same units. Units are tracked through multiplication and division.
Numbers are stored as decimals and addition and subtraction will use decimal (not binary) arithmetic. No guarantees on precision are given on the type or on other operations.