a rounded knob on something upright: the bulb-like ornaments of the tabernacle lampstand, and the shaped top of a pillar
This is a craftsman's word for a rounded swelling worked onto something that stands upright. It shows up in only twelve verses, and they fall into two settings: the golden lampstand of the tabernacle, and the top of a pillar in a building.
Ten of the twelve verses describe the lampstand. Exodus pictures it as a tree of hammered gold: a base, a central shaft, six branches, and along them cups shaped like almond blossoms, each cup paired with one of these knobs and a flower. One knob sits under each pair of branches, right where they leave the shaft, so it reads like a bud or a joint bulging out of a stem. Nothing is glued or fastened on. The knobs, the branches and the cups are all beaten out of one single piece of gold, which is why the text keeps saying they are "of one piece with it".
The other two verses are about buildings, and there the standard English word is "capital", the block that finishes off the top of a column. In Amos the Lord stands by the altar and gives the order to strike this piece so that the thresholds below shake, top and bottom of the building shuddering together. In Zephaniah, wild birds settle down for the night on these tops in a city that has been emptied out. For that kind of pillar top there is a more common Hebrew word (כֹּתֶרֶת), and this one steps in only twice.
What holds the two settings together is a shape rather than a place. In both, the word names the swelling at the top or at the joint of something standing up, so the same term can serve a piece of sacred furniture and a stone building without shifting its sense.