Wikipedia
In linguistics, extrametricality is a tool for prosodic analysis of words in a language. In certain languages, a particular segment or prosodic unit of a word may be ignored for the purposes of determining the stress structure of the word. For example, in a language like classical Latin, where polysyllabic words never have stress on their final syllables, and the position of stress in a word is determined by looking at the penultimate and antepenultimate syllables only, it simplifies the linguistic formulation of the stress-assignment rules of Latin to say that the final syllable of a polysyllabic word is invisible to rules which determine stress. Such invisibility is called "extrametricality" in linguistic terminology. This is purely a theoretical device — an extrametrical sound or syllable is not observed to have any special pronunciation, and extrametricality can only be determined by analyzing the prosodic patterns of the language as a whole.
Most typically, extrametricality affects one specified segment or prosodic unit ( mora, syllable, etc.) at the edge of a word (usually the end). Final-segment or final-mora extrametricality can be invoked to account for the phenomenon that word-final syllables often count as "lighter" than other syllables of the same rime type for purposes of determining the position of stress in a word.