Glowing is a harmful (negative) status effect in Minecraft Java Edition 1.21.
minecraft:glowing
Give Glowing II for 60 seconds (Java Edition 1.13+; duration in seconds, then amplifier where 0 = level I):
Remove the effect from the nearest player:
Glowing outlines the affected entity with a bright border visible through blocks, marking targets for everyone around.
Spectral arrows apply Glowing on hit, crafted with glowstone dust around an arrow. Ringing a village bell during a raid applies Glowing to raiders in range, which is the standard way to find the last hidden pillager.
Glowing is team information, not damage: use spectral arrows in PvP to keep a runner visible, and keep a bell near raid chokepoints. The outline color follows team colors, which matters on servers with scoreboard teams.
The effect ID for Glowing is minecraft:glowing. Java Edition 1.13+ /effect commands use this namespaced form. On Java 1.12.2 and earlier the numerical ID was 24.
Run /effect give @p minecraft:glowing 60 1 to get Glowing II for 60 seconds. The first number is the duration in seconds; the second is the amplifier (0 = level I, 1 = level II). Remove it with /effect clear @p minecraft:glowing.
Glowing is a harmful (negative) status effect in Minecraft Java Edition, based on its registry category (harmful).
Shoot them with spectral arrows, crafted from an arrow surrounded by four glowstone dust, or ring a bell during a raid to apply Glowing to nearby raiders. The outline shows through walls.
Tracking entities through blocks: locating the last raider of a wave, keeping PvP targets visible, and highlighting mobs in custom maps. It deals no damage and has no downside to the target besides visibility.