a device for varying the brightness of an electric light

a headlight with a low beam

(of a light, colour, or illuminated object) not shining brightly or clearly

the dim glow of the fire

not clearly recalled or formulated in the mind

dim memories

the matter was in the dim and distant past

stupid or slow to understand

you're just incredibly dim

such people are portrayed as being rather dim

make or become less bright or distinct

a smoky inferno that dimmed the sun

the lights dimmed and the curtains parted