Although vocabulary has conventionally been conceptualized as individual words, it has now become clear that much of lexis consists of sequences of words which operate as single units. Traditional approaches have long dealt with multi-word units (MWU), wherein a single meaning is attached to more than one word, for example, phrasal verbs (‘give up’), compounds (‘freeze-dry’), and idioms (‘burn the midnight oil’). But corpus-based research has shown that collocation (the tendency for words to occur together in discourse) extends far beyond the level of such MWUs. In fact, it appears quite common for longer sequences of words to pattern together. Some of these recur frequently enough to be treated as units in their own right, e.g. to make a long story short. Numerous terms have been coined to refer to this type of sequence, but the most commonly used are lexical chunks and lexical phrases.