Perplexity

Perplexity Definition 1 Given a sequence $X=x_1,x_2,\dots ,x_T$ of words and a language model $p$, the perplexity of the sequence can is defined as follows: $$\text{PPL}(X)=p(x_1,x_2,\dots ,x_T{)}^{-\frac{1}{T}}$$