The subset of language and vocabulary which one uses regularly. One's personal lexicon can show a variety of interesting things about them. People can tend to use specific constructions, specific words, words or phrases within constructions--each of these usages may be peculiar to that one person. For example, if one were to use the construction "said X" (as in the sentence "I bought said notebook at the store") on a regular basis, that construction could be said to be part of that person's personal lexicon. The use of this construction might then show that that person has some interest in strange grammatical forms, or that the society in which they normally function tends to use this type of construction.
One's personal lexicon is of great importance in the development of a group's cultural idiom. Each person in a given group has the power to introduce new words and usages into that group. If the other members of a group hear a new construction or phrasing enough, they may start to use it as well. This leads to the evolution of new idiomatic expressions, which only have new meaning to those in that group.
The term "lexicon" itself usually refers to a set of specific words, but most constructions use the same words in a specific manner, so I think they count as part of the personal lexicon. The inflection and usage of words matter just as much as the general meaning.
My own personal lexicon includes:
and a variety of others.