A CAT tool (computer-assisted translation tool) is software that splits a text into segments, shows each one next to its translation, and reuses earlier translations from a translation memory so that a human translator works faster and more consistently.
What a CAT tool does
Segments the text. The source document is cut into sentences or paragraphs, and the translator works one segment at a time, in a two-column editor.
Suggests earlier translations. Each segment is looked up in the translation memory. An identical sentence comes back as a 100% match, a similar one as a fuzzy match.
Keeps terminology consistent. A termbase shows the approved translation of each term as the translator types.
Protects the layout. Text is extracted from Word, InDesign, HTML or software strings into a bilingual file, often XLIFF, and put back into the original format at the end.
Checks quality and counts words. Built-in QA catches missing numbers or tags, and a CAT analysis counts the words by match band for the quote.
What a CAT tool is not
A CAT tool is not machine translation. DeepL and Google Translate are MT engines: they translate automatically, and most CAT tools can plug them in as one more source of suggestions. In a CAT tool, the translator decides what goes into every segment.
Common CAT tools
Trados Studio, memoQ, Phrase, Wordfast Pro and Smartcat are the best known paid options; OmegaT (desktop) and MateCat (browser) are free and open source. Our CAT tools comparison covers prices, Mac support and file compatibility.
Example
A manual has 2,000 words, and 300 of them repeat sentences the translator already translated last year. In a CAT tool, those sentences appear pre-filled from the translation memory: the translator only checks them in context, and translates the rest. The same TM then keeps both manuals consistent.