Definitions
1.adjectiveNot bright or colorful."The lighting was too dim for me to make out his facial features."
2.adjectiveNot smart or intelligent."He may be a bit dim, but he's not stupid."
3.adjectiveIndistinct, hazy or unclear."His vision grew dimmer as he aged."
4.adjectiveDisapproving, unfavorable: rarely used outside the phrase take a dim view of.
5.adjectiveLessened, reduced"The diminished Roman Empire never recovered from the sack of Rome."
6.adjectiveMade to seem less important, impressive, or valuable"She felt diminished by the report card."
7.adjectiveReduced by a semitone
8.nounA rheostat that is used to vary the intensity of a domestic electric light
9.nounA switch used to select between the low and high headlamp beam on a road vehicle. (usually as "dimmer switch", primarily in North America; elsewhere "dipswitch" or "dipper switch")