extrametrical

extrametrical, in prosody, exceeding the usual or prescribed number of syllables in a given metre. Also, in reference to a syllable or syllables not counted in metrical analysis. In the following final couplet from a sonnet by William Shakespeare, the ending syllables are extrametrical:

Yet him for this my love no whit disdaineth;
Suns of the world may stain when heaven’s sun staineth.