Final answer:
The term for projecting support at the end of an arcade or colonnade is a buttress, not to be confused with a pilaster which is decorative.
Step-by-step explanation:
The term for a pier or pilaster projecting from a wall as a support for an arch or lintel, especially at the termination of an arcade or colonnade, is known as a buttress. It is a projecting structure that reinforces or stabilizes a wall or building. On the other hand, a pilaster is a rectangular column that projects partially from the wall it is attached to but serves a decorative purpose rather than structural support. An arch itself is composed of wedge-shaped stones called voussoirs with the topmost voussoir being the keystone, crucial for maintaining the arch's stability. When arches are placed side by side, they form what is known as an arcade.