Definition
A condition in which the leading (line spacing) is less than the type size, so that the vertical distance from baseline to baseline is smaller than the body height of the type. For example, 12 pt type set on 11 pt leading (12/11) has negative leading. This is sometimes used with display type and caps, where ascenders and descenders do not collide.
Source
— Robert Bringhurst — p. 327:
A condition in which the leading is less than the type size.
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