The ornamental design for a defibrillator, substantially as shown and described.
FIG. 1 is a front perspective view of the defibrillator.
FIG. 2 is a front view thereof.
FIG. 3 is a back view thereof.
FIG. 4 is right side view thereof.
FIG. 5 is left side view thereof.
FIG. 6 is top view thereof.
FIG. 7 is bottom view thereof; and,
FIG. 8 is rear perspective view thereof.
The broken lines immediately adjacent the shaded areas represent the bounds of the claim, while all other broken lines are directed to environment and are for illustrative purposes only; the broken lines form no part of the claimed design.