The leading is the amount of space added to increase line spacing (the height of the space between two baselines).

HISTORY

The word refers to the strips of lead used to separate lines of type with in the era of metal-type printing. The line spacing between two lines of 12 pt text set solid was exactly 12 pt. When 2 pt piece of lead was added between those lines, the line spacing value was increased to 14 pt. But the leading value was not 14 pt, instead it was 2 pt.

TODAY

Increasing line-spacing values in InDesign or in CSS isn’t per se “leading,” it is a “line-spacing increase.” Although, it is quite common to say “increase the leading” while you are technically increasing the line spacing.