A character set (sometimes referred to as code page) is a collection of characters that are associated with a sequence of natural numbers in order to facilitate the storage of text in computers and the transmission of text through telecommunication networks.

A language code is a code that assigns letters or numbers as identifiers or classifiers for languages. Language codes used in TMX files must comply with the rules defined by BCP 47 - Tags for Identifying Languages

A Subscription key is a short text code that lets you register and use a software program during a pre-established time period.

Translation Memory eXchange (TMX) is an open standard originally published by LISA (Localization Industry Standards Association). The purpose of TMX is to allow easier exchange of translation memory data between tools and/or translation vendors with little or no loss of critical data during the process.

The language of a document that is to be translated.

Translation Memory (TM) is a language technology that enables the translation of segments (paragraphs, sentences or phrases) of documents by searching for similar segments in a database and suggesting matches that are found in the databases as possible translations.

A translation unit (represented by the <tu> element in a TMX file) contains a piece of source text and optional translation and metadata related to that piece of text.