Question

reflect (object) across line (name).
reflect a across line m.
__________ is a rigid transformation that takes a point
to another point so that the directed
____________________ from the
original point to the image is __________ to the given
line segment and has the same __________ and
__________.
translate (object) by the directed line segment (name
or from point to point).
translate a by the
directed line segment v.

Explanation:

The definition describes a reflection, a rigid transformation. A reflection maps a point to its mirror image over a line, where the segment connecting the original point and its image is perpendicular to the line of reflection and bisected by it (so the line is the perpendicular bisector, meaning the segment has equal length on both sides of the line).

Answer:

Reflection; line segment; perpendicular; length; is bisected by the line (or "midpoint on the line")

Note: The last blank may vary slightly by curriculum, but common completions for the definition are:
Reflection is a rigid transformation that takes a point to another point so that the directed line segment from the original point to the image is perpendicular to the given line segment and has the same length and its midpoint lies on the line.