Both. FAA AIM 2-3-4:
b. Taxiway Centerline.
1. Normal Centerline. The taxiway centerline is a single continuous yellow line, 6 inches (15 cm) to 12 inches (30 cm) in width. This provides a visual cue to permit taxiing along a designated path. Ideally, the aircraft should be kept centered over this line during
taxi. However, being centered on the taxiway centerline does not guarantee wingtip clearance with other aircraft or other objects.
Note that SMGCS lighting (green lights) will overlie the yellow lines (AC 120-57A).