Dead, stuck or hot pixel?
A dead pixel stays black on every color because it no longer gets power. A stuck pixel is stuck on one color, usually red, green or blue, and is easiest to see on a contrasting background. A hot pixel is always lit and shows up as a bright dot on black. Stuck pixels can sometimes be revived by gently massaging the spot or running a fast color-flashing video for a while; dead pixels usually cannot.
Check before the return window closes
Run this test as soon as you buy a new monitor, laptop or phone. Many stores and manufacturers only replace screens with a certain number of bad pixels, so take a photo of any you find and contact them within the return period.
Backlight bleed and banding
In a dark room, the black screen reveals light leaking from the edges of LCD panels (backlight bleed) or a glow in the corners of IPS panels. The gradient shows banding: steps between shades that should be smooth, common on 6-bit or heavily compressed displays.