A termbase is a terminology database in which each entry holds one concept with its approved terms in several languages, plus optional definitions, notes and forbidden variants, so that every translator on a project uses the same words.
A termbase is organised by concept, not by word. The entry for one concept can hold:
A simple glossary (a two-column list of source and target terms) is the lightest form of termbase, and many clients send exactly that as a spreadsheet.
A translation memory stores whole sentences and their translations, and suggests them when similar sentences come back. A termbase stores single terms and phrases, and the CAT tool shows the approved translation whenever the term appears in a segment, whatever the rest of the sentence says. Most projects use both.
Each CAT tool keeps its termbases in its own format: Trados uses MultiTerm (.sdltb), and memoQ has its own termbases. The standard exchange format is TBX (TermBase eXchange, ISO 30042), and most tools also import and export CSV or Excel. Our post on XLIFF, TMX and TBX compares the formats.
A medical device client's termbase says that "device" must be translated as "dispositivo" in Spanish, and that "aparato" is forbidden. While translating, the CAT tool shows "dispositivo" next to every segment that contains "device". Before delivery, a terminology check flags any segment where "device" was translated some other way.
You can run that check without a CAT tool: TraduQ's free QA checker imports a glossary from CSV or Excel, including forbidden terms, and checks XLIFF, SDLXLIFF, MQXLIFF and TMX files against it in your browser. The QA checking guide shows how.