Halliday & Matthiessen (1999: 531):
Lexical cohesion refers to cohesion that is brought about by lexical means: choosing a word that is related in a systematic way to one that has occurred before. The range of semantic relations that can create cohesion in this way is very wide; but there are five principal conditions under which it occurs. These are: repetition, where the speaker simply repeats the same word; synonymy/antonymy, where a word is chosen that is similar or opposed in meaning; hyponymy/meronymy, where a word is chosen that is related by 'kind of' or 'part of — either vertically, like melon ... fruit or car ... wheel, or horizontally, like melon ... plum, or wheel ... mudguard; and collocation, which does not necessarily imply any particular semantic relationship but means simply that a word is chosen which is regularly associated with a previous one, like aim coming shortly after target, such that a resonance is felt between the two.