A file that stores translations (e.g. a PO file) — although it could also be used to refer to other ways of storing translations.
Contains a number of Translation Units, which contain messages.
At the simplest level contains a single source string (the original message) and a single target string (the translated message).
XLIFF refers to this as a unit, Gettext calls it a message or string. Some industry tools talk of segments. To maintain consistency we refer to string in the GUI and unit in the code.
Monolingual formats (like .properties, OpenOffice SDF, DTD, HTML, etc.) only contain a source strings.
However when handling plurals the source may actually contain different variants of a message for different plural forms (e.g. in English, the singular and plural), and the target as well (the number of variants in source and target strings are often different because different languages handle plurals differently).