* Tetrahedron: A three-dimensional shape with four triangular faces.
* Tetrad: A group of four.
* Tetrameter: A line of poetry containing four metrical feet.
* Tetrapod: A vertebrate animal with four limbs.
* Tetracycline: A type of antibiotic.
* Tetraethyllead: A formerly widely used anti-knock agent in gasoline.
* Tetrachord: A musical interval consisting of four notes.
* Tetralogy: A series of four related literary works.
* Tetrahydrocannabinol (THC): The psychoactive compound in cannabis.
* Tetranucleotide: A molecule composed of four nucleotides.