The analysis and graphic display of a line's rhythm performed by scanning the line to determine its metrical categorization, e.g., iambic trimeter, trochaic octameter, etc., as a way of describing the rhythmical pattern of a poem. Scansion will also show the variations in the meter and the deviations from it, if there are any.
Sidelight: Scansion accounts for syllabic accents and slacks, but does not always differentiate between the relative "weights" of stress, one of the means by which a skillful poet modulates the rhythm for effect.
Sidelight: The scanning process employs symbols on and above the lines to identify the foot divisions, their arsis and thesis, and any internal caesuras the line may contain. Unfortunately, the symbols for the arsis and thesis cannot be shown in this example:
One shade / the more, || one ray / the less,
Had half / impair'd / the name / less grace
Sidelight: By definition, scansion entails the scanning of one line at a time. Roving over, a term suggested for the scanning of Hopkins' sprung rhythm, is a process in which scansion is continued from one line to the next without interruption.
Sidelight: Individual judgments often play a part in the scansion process, since the divisions between feet may be subject to differences of interpretation.

This definition is in context to Poetry. See more contextual defintions for Scansion.