A cognate is a word derived from the same root as another word. Cognates are words that have a common origin (source). They may happen in a language or in a group of languages.

Example One:

'composite', 'composition' and 'compost' are cognates in the English language, derived from the same root in Latin 'componere' meaning 'to put together'.

Example Two:

the word 'composition' in English and the word 'composición' in Spanish and similar words in French, Italian and Portuguese are cognates because they all come from the same root.

The general rule is that cognates have similar meanings and are derived from the same root (origin). A fine example is the word for night in almost all Indo-European languages: