Meta- (from the Greek preposition μετά = "after", "beyond", "adjacent", "self", also commonly used in the form μετα- as a prefix in Greek, with variants μετ- before vowels and μεθ- "meth-" before aspirated vowels), is a prefix used in English (and other Greek-owing languages) to indicate a concept which is an abstraction from another concept, used to complete or add to the latter.
In epistemology, the prefix meta- is used to mean about (its own category). For example, metadata are data about data (who has produced them, when, what format the data are in and so on). Also, metamemory in psychology means an individual's knowledge about whether or not they would remember something if they concentrated on recalling it. Furthermore, metaemotion in psychology means an individual's emotion about his/her own basic emotion, or somebody else's basic emotion.
Another, slightly different interpretation of this term is "about" but not "on" (exactly its own category). For example, in linguistics a grammar is considered as being expressed in a metalanguage, or a sort of language for describing another language (and not itself).
Any subject can be said to have a meta-theory which is the theoretical consideration of its meta-properties, such as its foundations, methods, form and utility.
In Greek, the prefix meta- is generally less esoteric than in English; Greek meta- is equivalent to the Latin words post- or ad-. The use of the prefix in this sense occurs occasionally in scientific English terms derived from Greek. For example: the term Metatheria (the name for the clade of marsupial mammals) uses the prefix meta- merely in the sense that the Metatheria occur on the tree of life adjacent to the Theria (the placental mammals).
Meta is also gaining currency as an adjective, as well as a prefix, as in the work of Douglas Hofstadter (see below).