Synonyms: line-height, line leading, type spacing, baseline spacing, interline spacing
Leading is the space (vertical distance) between each line of text in a paragraph and commonly measured as a percentage of font size.
«a fine line»
Often wrong leading can make a block of text less readable. Thereby, 'beautiful' fonts can become ugly when they're unreadable simply because frustration always trumps aesthetics. Your users will immediately notice that there’s something bad with your text, even if they can’t put their finger on what it is.