Literally an animal horn; symbolically, power and glory. God 'exalts the horn' of his anointed (1 Sam 2:10). Moses' face 'shone' (qaran from same root) after meeting God (Exod 34:29).
Horns on the altar were sacred (sanctuary for those who grabbed them). Lifting or breaking someone's horn meant rising or falling power. Daniel's horns in apocalyptic represent successive empires.
Luke 1:69 ('horn of salvation') directly echoes the OT horn idiom. The 'horn of David's house' is Messiah — the one whose power God has exalted. Horn symbolism in Revelation (the Lamb with seven horns) draws on this tradition.