Avoidance speech, or "mother-in-law languages", is a feature of many Australian Aboriginal languages and some North American languages and Bantu languages (called ukuhlonipha, "to respect", in Zulu, for example) of Africa whereby in the presence of certain relatives it is taboo to use everyday speech style, and instead a special speech style must be used.
Avoidance speech styles tend to have the same phonology and grammar as the standard language they are a part of. The lexicon, however, tends to be smaller than in normal speech, since it only needs to be used when conversation with the taboo relatives is necessary.
For instance, in Dyirbal there is the regular speech style (called Guwal) and the avoidance style Dyalngui consisting of a special set of lexical items that are substituted for Guwal words in the presence of opposite-sex parents-in-law, opposite-sex children-in-law, and opposite-sex cross-cousins. These words are fewer, however, and their meanings tend to be much more generic, e.g. the Dyalngui verb bubaman does service for the Guwal verbs baygun "shake", dyindan "wave" and banyin "smash".