A person's vocabulary is the set of words within a language that are familiar to that person. A vocabulary usually develops with age, and serves as a useful and fundamental tool for communication and acquiring knowledge. Acquiring an extensive vocabulary is one of the largest challenges in learning a second language.

Vocabulary is commonly defined as "all the words known and used by a particular person".^[1] Knowing a word, however, is not as simple as simply being able to recognize or use it. There are several aspects of word knowledge which are used to measure word knowledge.

The first major distinction that must be made when evaluating word knowledge is whether the knowledge is productive (also called achieve) or receptive (also called receive) and even within those opposing categories, there is often no clear distinction. Words that are generally understood when heard or read or seen constitute a person's receptive vocabulary. These words may range from well known to barely known (see degree of knowledge below). A person's receptive vocabulary is the larger of the two. For example, although a young child may not yet be able to speak, write, or sign, he or she may be able to follow simple commands and appear to understand a good portion of the language to which he or she is exposed. In this case, the child's receptive vocabulary is likely tens, if not hundreds of words but his or her active vocabulary is zero. When that child learns to speak or sign, however, the child's active vocabulary begins to increase. It is possible for the productive vocabulary to be larger than the receptive vocabulary, for example in a second-language learner who has learned words through study rather than exposure, and can produce them, but has difficulty recognizing them in conversation.

Productive vocabulary, therefore, generally refers to words which can be produced within an appropriate context and match the intended meaning of the speaker or signer. As with receptive vocabulary, however, there are many degrees at which a particular word may be considered part of an active vocabulary. Knowing how to pronounce, sign, or write a word does not necessarily mean that the word has been used to correctly or accurately reflect the intended message of the utterance, but it does reflect a minimal amount of productive knowledge.

Within the receptive–productive distinction lies a range of abilities which are often referred to as degree of knowledge. This simply indicates that a word gradually enters a person's vocabulary over a period of time as more aspects of word knowledge are learnt. Roughly, these stages could be described as:

The differing degrees of word knowledge imply a greater depth of knowledge, but the process is more complex than that. There are many facets to knowing a word, some of which are not hierarchical so their acquisition does not necessarily follow a linear progression suggested by degree of knowledge. Several frameworks of word knowledge have been proposed to better operationalise this concept. One such framework includes nine facets: