Lexis refers to the total stock of words and word-like units in a language, or in a speaker's repertoire. In ELT, "lexis" is preferred over "vocabulary" because it captures a broader reality: language is not just single words but multi-word units, including collocations, formulaic sequences, phrasal verbs, and idioms, stored and processed as wholes.
"Vocabulary" traditionally implies lists of single words with definitions. "Lexis" signals a shift toward how words actually behave in use: their collocational profiles, grammatical patterns, register restrictions, and multi-word combinations. Michael Lewis's Lexical Approach (1993) made this distinction central to ELT methodology, arguing that "language consists of grammaticalized lexis, not lexicalized grammar."
The mental lexicon is the speaker's internal dictionary: a network of entries connected by form, meaning, collocation, and association. It is not a static list but a dynamic system:
Paul Nation (2001, Learning Vocabulary in Another Language) provided the most influential framework for what it means to "know" a word, organized across three dimensions:
| Dimension | Receptive | Productive |
|---|---|---|
| Form | Recognise spoken/written form | Produce spoken/written form |
| Meaning | Retrieve meaning from form | Express meaning with correct form |
| Use | Recognise collocations, register, frequency | Produce natural collocations, appropriate register |
Each dimension has receptive and productive knowledge, yielding a rich, multi-layered picture. A learner may "know" a word receptively (recognise it in reading) long before they can use it productively with natural collocations.