The ornamental design for a lightbulb, as shown and described.
FIG. 1 is a perspective top view of a light bulb showing our new design, wherein the downlight portion of the surface of the light bulb is transparent in appearance.
FIG. 2 is a perspective bottom view of the light bulb.
FIG. 3 is a top plan view of the light bulb.
FIG. 4 is a bottom plan view of the light bulb.
FIG. 5 is a side elevational view of the light bulb.
FIG. 6 is a side, cross-sectional view of the light bulb taken from the line 6-6 in FIG. 5.
FIG. 7 is a side view of a minimally modified variation of the light bulb of FIG. 1.
FIG. 8 is a side view of a minimally modified variation of the light bulb of FIG. 1.
FIG. 9 is a side view of a minimally modified variation of the light bulb of FIG. 1.
FIG. 10 is a side view of a minimally modified variation of the light bulb of FIG. 1; and,
FIG. 11 is a side view of a minimally modified variation of the light bulb of FIG. 1.
The broken lines in the drawings depict parts of the light bulb that form no part of the claimed design.