Glossary Term Definition
Type set without additional leading — the line spacing equals the type size exactly.
Definition
Type set solid has no additional space between lines beyond the body size of the type itself — that is, the leading equals the type size. For example, 12-point type set solid is set 12/12 (twelve on twelve). Setting type solid was standard in the days of hand composition, when the physical body of each piece of type determined the minimum line spacing. In modern practice, most text benefits from at least one or two points of additional leading, and setting solid is generally reserved for display sizes or deliberate typographic effects where a dense, compact texture is desired.
Source
No access — Robert Bringhurst — p. 345:
Set without additional lead, or with leading equal to the type size.