The ornamental design for a pillow, as shown and described.
FIG. 1 is a top front perspective view of a pillow;
FIG. 2 is a top plan view thereof;
FIG. 3 is a bottom plan view thereof;
FIG. 4 is a front elevational view thereof;
FIG. 5 is a rear elevational view thereof;
FIG. 6 is a left side elevational view thereof; and,
FIG. 7 is a right side elevational view thereof.
The outer perimeter of broken line of the broken line mesh feature forms the boundary of the claim. The broken lines and all trapped spaces within illustrate portions of the pillow that form no part of the claim.