Translation is the interpretation of the meaning of a text in one language (the "source text") and the production, in another language (the "target language"), of an equivalent text (the "target text," or "translation") that communicates the same message.

Translation must take into account constraints that include context, the rules of grammar of the two languages, their writing conventions, and their idioms.

Traditionally translation has been a human activity, though attempts have been made to computerize or otherwise automate the translation of natural-language texts (machine translation) or to use computers as an aid to translation (computer-assisted translation).

Perhaps the most common misconception about translation is that there exists a simple "word-for-word" correspondence between any two languages, and that translation is therefore a straightforward mechanical process. On the contrary, every language is a historically-evolved self-contained system, and historically-determined differences between languages may dictate differences of expression.

Interpreting, or "interpretation," is the intellectual activity that consists of facilitating oral or sign-language communication, either simultaneously or consecutively, between two or among three or more speakers who are not speaking, or signing, the same language.

The words "interpreting" and "interpretation" both can be used to refer to this activity; the word "interpreting" is commonly used in the profession and in the translation-studies field to avoid confusion with other meanings of the word "interpretation."

Not all languages employ, as English does, two separate words to denote the activities of written and live-communication (oral or sign-language) translators.

If one text is a translation of another, a back-translation is a translation of the translated text back into the language of the original text, made without reference to the original text. In the context of machine translation, this is also called a "round-trip translation."

Comparison of a back-translation to the original text is sometimes used as a quality check on the original translation, but it is certainly far from infallible and the reliability of this technique has been disputed.