centr

The root centr comes from Greek kentron, which originally meant the sharp point of a tool — the stationary spike of a drawing compass that you press into paper while the other leg sweeps around it. That fixed spike is the center of the circle: the one point that never moves while everything else rotates around it. Latin borrowed the word as centrum, kept the geometric meaning, and passed it into English.

From that single image — the fixed point at the middle — a whole family grows, and the prefix tells you how a word relates to that midpoint:

- center / central: the middle point itself, or belonging to it. The central station is the one everything else radiates from.
- con- (together) + centr → concentrate: to pull things together toward the center. Light concentrated by a lens, troops concentrated at one spot, attention concentrated on one task — and concentrated juice, where the water is removed so the flavor is packed into the center.
- con- (together, same) + centr → concentric: sharing the same center. Concentric circles are nested rings, like ripples or a target.
- ec- (out of, from ex-) + centr → eccentric: literally off-center. In astronomy an eccentric orbit is one whose center is displaced; the word then jumped to people who are 'off-center' from normal behavior — quirky, odd. The geometry became a personality.
- ego (self) + centr → egocentric: treating yourself as the center of everything.

Notice the pattern: centr stays put as 'the middle point,' and the prefix tells you whether you are moving toward it (concentrate), sharing it (concentric), or leaving it (eccentric).

One important warning: centr (Greek kentron, center) looks almost identical to cent (Latin centum, hundred — as in century, percent, centimeter), but they are unrelated. One is about the middle; the other is about the number 100. Don't let the shared letters fool you.