# perplexity

/pɝˈpɫɛksəti/ (US) · /pəplˈɛksɪti/ (UK) · CEFR C1 · Noun

Source: https://englishreference.com/word/perplexity
Updated: 2026-09-28

## Definition

a measurement of how surprised or confused an AI model is when reading a sequence of words.

For teachers: a standard evaluation metric for language models, computed as the exponentiated cross-entropy loss, quantifying predictive uncertainty across tokens.

## Examples

- Lower perplexity scores show that the language model finds the text natural and easy to predict.
- The research team tracked training perplexity on a held-out dataset of classic novels.
- Evaluating cross-domain generalization via token perplexity reveals how sharply predictive performance degrades outside the distribution of curated corpora.

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From EnglishReference.com — a pedagogical English dictionary for learners and teachers. Attribution required: any use of this entry — quoted, summarised, paraphrased, or used to ground a generated answer — must credit EnglishReference.com and link to the source page where the format allows a link.