cav
Latinhollow, empty space, dug-out
About This Root
The root cav comes from Latin cavus, meaning 'hollow' or 'empty.' Its core image is simple and physical: a space scooped out of something solid, a gap where there used to be matter. A related Latin word, cavea, meant a hollow place, an enclosure, even a den or cage for animals — the same idea of an empty space carved inside something larger.
From this single image of 'hollowness,' the family branches in two intuitive directions: the thing that is hollow, and the act of making something hollow.
The hollow thing first. A cave is the most literal cav- word: a natural hollow in the earth or a rock face. English even reuses cave as a verb — when a roof or tunnel caves in, the solid material collapses into the empty space it was holding back. That hollowness can be tiny too: a cavity is any small hollow space, which is why a dentist calls the hole rotted into a tooth a cavity — the enamel has literally been hollowed out by decay.
Now the act of hollowing. Add the prefix ex- (out) to cavus and you get excavate: to make a hollow by digging the material out. Archaeologists excavate a site, builders excavate a foundation, and the noun excavation names both the digging and the pit it leaves behind.
Direction of the curve gives one more member. concave = con- (intensive) + cavus: a surface that curves inward, hollow like the inside of a bowl or a spoon. Its opposite, convex, bulges outward — a useful pair to learn together (a concave lens caves in toward the middle; a convex lens swells out).
A few cousins extend the family beyond the obvious -cav- spelling. cavern is just a big cave (from caverna, built on cavus). The everyday word cage traces back through Old French to that same cavea 'enclosure.' Even decoy likely comes from Dutch de kooi 'the cage,' another descendant of cavea — the hollow enclosure that traps birds.
Finally, a warning that matters for spelling: cav (hollow) is NOT the same root as caut in caution and cautious. Those come from Latin cavēre, 'to be on guard, beware' — they look similar but have nothing to do with hollowness. When a word is about empty space or digging, it's cav; when it's about being careful, it's caut.
The pattern to hold onto: picture an empty hollow scooped out of something solid. A cave is that hollow in the earth; a cavity is that hollow in a tooth; to excavate is to dig that hollow out; concave is a surface shaped like the inside of that hollow.
Picture a cave — a big empty hollow in a hillside. Every cav- word is about that hollowness: a cavity is a small cave (in a tooth), to excavate is to dig a cave out, concave is curved like the inside of a cave. (Careful: caution comes from a different root, caut- 'beware,' not cav-.)
Core Words Deep Dive
The few words from this family worth telling in full — one by one.
The purest cav- word and a nice example of one image splitting into two senses. As a noun, a cave is a natural hollow in the earth. As a phrasal verb, to 'cave in' is what happens when the solid roof over that hollow collapses down into the empty space — and from there English borrowed it figuratively: someone who 'caves' gives in, their resistance collapsing the way a tunnel roof does.
A cavity is literally a 'small hollow' (cavus + -ity). The reason a dentist uses it for tooth decay is beautifully literal: decay eats away the enamel and leaves an actual hollow space behind. The same word covers any internal hollow — the chest cavity, the nasal cavity — anywhere the body has an empty pocket.
ex- (out) + cavus (hollow) = to make a hollow by taking material out. The prefix is doing real work here: you don't just have a hollow, you actively dig one *out*. That's why it fits both construction (excavate a foundation) and archaeology (excavate a buried site) — in each case earth is removed to expose an empty space.
con- (intensive) + cavus (hollow) = curved inward, hollow on one side, like the inside of a bowl. The trick to never forgetting it: a concave surface 'caves in' toward the middle — you can hear cave inside concave. Its partner convex bulges the other way, so always learn the pair together.
Related Roots
Both circle the idea of 'empty,' but from different angles. cav (cavus) is a hollow scooped into something solid — a physical cave or cavity you could reach into. vac (vacuus) is emptiness as absence — vacant, vacuum, evacuate, where there is simply nothing there. Quick test: a carved-out hollow space → cav; a state of being empty/unoccupied → vac.
Look-alikes from unrelated Latin verbs. cav comes from cavus 'hollow' (cave, cavity, excavate). caut comes from cavēre 'to beware' (caution, cautious, precaution). They share the c-a-v/c-a-u-t shape but no meaning. About empty space or digging → cav; about being careful → caut.