The mental lexicon stores words and phraseologisms as pairs of expression and meaning. In speech perception, the expression side is accessed and matched with the meaning; in speech production, the mapping takes the reverse direction.

The mental lexicon is part of long-term memory. More specifically, it is part of procedural, not of declarative memory. This means that items are learnt and stored not as pieces of knowledge, but as procedures to associate both sensory input and motoric output with mental operations. For this kind of knowledge to be learnt, it does not suffice to memorize what has been read or heard. It must be rehearsed in order to be learnt.

With three years of age, the child can segment speech input and assign a separate word its meaning. Just as in general with language acquisition, comprehension precedes production. Only a minor part of the vocabulary is taught by ostension and definition. The child assigns the bulk of new words their meanings by inference based on information from the semantic and pragmatic context. This works essentially by the same mechanisms used by an adult who is reading a text and stumbles over an unknown word. The meaning thus construed may be imprecise on first occasion and corrected and refined upon repetition.

The law that speed and persistence of procedural learning depend on frequency of practice is particularly evident in the learning of vocabulary. Beyond this general law, children from three years of age on are better in this than adults; i.e. they need fewer rehearsals to ensure retention.

In the holophrastic phase, the child appears to be learning words. However, these are one-word utterances stored holistically. A stock of vocabulary is built when the child can segment utterances into words, from three years of age on. The identity and category (noun or verb) of the first words depend entirely on the social context in which the child is learning. At any rate:

• The first words are lexemes such as milk and drink, not grammatical formatives such as some or will (although the latter are much more frequent).
• The first words denote things and situations present in the speech situation, such as milk and drink, rather than absent or imagined entities such as bell and chime.
• Their meanings are concrete, such as car and broken, rather than abstract, like trip or check.

By an age of two years, the normal child knows more than 200 utterances of word size. For adult speakers, estimates of the size of their mental lexicon vary enormously because the criteria are not standardized. At any rate, the English vocabulary of an adult US-American ranges in some tens of thousands of items.