The ornamental design for an impeller for a pump, as shown and described.
FIG. 1 is a front perspective view of an impeller for a pump showing the new design;
FIG. 2 is a rear perspective view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a right side view thereof;
FIG. 7 is a top view thereof; and,
FIG. 8 is a bottom view thereof.
The design includes a mirror image of the embodiment depicted.
The drawings include shadows and reflections, which indicate contour and not surface ornamentation.
Those portions of the drawings shown in broken lines, as well as the interior cylindrical surfaces between those circular broken lines, depict portions of the impeller for a pump that form no part of the claimed design.