Tetra is a prefix from Greek meaning “four.” You see it in many scientific and everyday words where the number four matters. A tetrahedron is a three-dimensional shape with four triangular faces, like a pyramid with a triangular base. Tetrapods are animals with four limbs: frogs, lizards, dogs, and humans are all tetrapods, even though whales and snakes evolved from four-limbed ancestors and lost their legs over millions of years.

In chemistry, tetrachloride means a compound with four chlorine atoms. Musicians talk about tetrachords, groups of four notes that form part of a musical scale.

The word tetra can also be a noun. It refers to small, colorful freshwater fish popular in home aquariums. Neon tetras have bright blue and red stripes and swim in groups called schools.

When you spot “tetra” in a word, start counting to four: it's almost certainly telling you something comes in fours or has four parts.