Morpheme
A morpheme was the smallest linguistic unit within a word that could carry a meaning.
In 3189, after Ensign Sylvia Tilly suggested a dark matter interface for the spore drive, Lieutenant Commander Paul Stamets informed her that dark matter was comprised of subatomic particles and the mycelial network was a subspace domain. He then added, "The prefix 'sub' being the only morpheme that remotely connects those two ideas." (DIS: "Forget Me Not")