Counterpoint technique in which the interval direction is changed. Any ascending interval becomes a descending interval and vice versa. An example from Bach's Invention BWV 772.

intervals in original motive	2nd ascending	2nd ascending	2nd ascending	3rd descending	2nd ascending	3rd descending
intervals in inverted motive	2nd descending	2nd descending	2nd descending	3rd ascending	2nd descending	3rd ascending

It looks like a reflection in a mirror. This is why it is sometimes called mirror inversion:

The Quarendo invenietis canon from Bach's Musical Offering is an example of a canon by inversion or contrary motion:

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