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 … synonymy/antonymy … hyponymy/meronymy … and collocation …