for: Neuro-linguistic programming

The word comes from lead strips that were put between set lines. When type was set by hand in printing presses, slugs or strips of lead (reglets) of appropriate thicknesses were inserted between lines of type to add vertical space, to fill available space on the page.

Text set "solid" (no leading) appears cramped, with ascenders almost touching descenders from the previous line. The lack of white space between lines makes it difficult for the eye to track from one line to the next, and hampers readability.

The following block of text has no leading:

Typography (Greek: typos "form", graphein "to write") is the art and technique of setting written subject matter in type using a combination of typeface styles, point sizes, line lengths, line leading, character spacing, and word spacing to produce typeset artwork in physical or digital form.

This block of text set with 50% leading is easier to read:

Typography (Greek: typos "form", graphein "to write") is the art and technique of setting written subject matter in type using a combination of typeface styles, point sizes, line lengths, line leading, character spacing, and word spacing to produce typeset artwork in physical or digital form.

This block of text at 100% leading is also easy to read, but makes less efficient use of vertical page space:

Typography (Greek: typos "form", graphein "to write") is the art and technique of setting written subject matter in type using a combination of typeface styles, point sizes, line lengths, line leading, character spacing, and word spacing to produce typeset artwork in physical or digital form.

In CSS, leading is implemented by the line-height property.