The second (symbol: s) is the base unit of time in the International System of Units (SI).

Originally defined as 1⁄86400 of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds each.

In construction plant fitting, and your Phase 2 course, the second is also part of other units of measurement such as meters per second for velocity, m/s.

The second, s, on its own is a base unit. When the second is used in conjunction with other units of measurement, like with distance to give speed or velocity, the new unit of measurement, m/s, is known as a base derived unit of measurement - it is made up of two base units, m and s, put together.