מִזְבֵּחַ mizbeach altar. These were of two main types:

  1. large structures often outside the entrance to the temple;
  2. smaller stone pillars with a flat surface which curved up at the corners ("horned altars") found inside the building.
Sacrifices were offered there, usually by burning.
Our information on altars is both biblical (from both the laws of Leviticus and the stories of the historical books) and archaeological.

Amos references in 2:8 & 9:1 by the physical actions involved suggest the larger external sort. While in 3:14 both types are in view (plural then specification of a singular horned altar).