UNK, unk, <unk> are variants of a symbol in natural language processing and machine translation to indicate an out-of-vocabulary word.

Many language models do calculations upon representations of the \(n\) most frequent words in the corpus. Words that are less frequent are replaced with the <unk> symbol.

This is what such a transformation might look like. The below is an example of a source document in a corpus with common English words.

Today I’ll bake; tomorrow I’ll brew, Then I’ll fetch the queen’s new child, It is good that no one knows, Rumpelstiltskin is my name.

Every word in the above quote is common in English, except for Rumpelstiltskin, which is replaced as following:

Today I’ll bake; tomorrow I’ll brew, Then I’ll fetch the queen’s new child, It is good that no one knows, <unk> is my name.