the capital of a pillar: the ornamented crown that finishes off the top of a column.
This is a builder's word, not an everyday one. It names the decorated piece that sits on the head of a column, the crown that caps a pillar. Almost every time it appears, it is describing the two enormous bronze pillars, named Jachin and Boaz, that stood at the entrance of Solomon's temple.
Those two capitals were showpieces. The account says each stood about five cubits tall, was wrapped in bronze lattice like netting, ringed with two rows of pomegranates, and finished with lily-shaped work; the rounded, bowl-shaped part of each capital is even given its own name in the text. The word travels with the pillars through the whole story: it appears in the building of the temple and again centuries later when Babylon tore the pillars down and carried the bronze away.
It comes from the same root as the ordinary word for a king's crown, כֶּתֶר. Both picture something that sits on top and completes what is beneath: one crowns a royal head, the other crowns a pillar's head. This word, though, never leaves the world of buildings.
Just once it steps away from the great pillars. In 1 Kings 7:31 it describes the rounded rim at the top of the wheeled bronze stands that carried the temple basins, the same idea of a crowning top-piece applied to a different object.