adj

• (dated) Having a single structure; not composite or complex; undivided, unitary.
• (anatomy, historical) Of an eye: (supposedly) having pigment on only the posterior surface of the iris and not the anterior surface, and thus appearing blue; this was later found to be inaccurate, as eye colour is due to the amount of pigment in the anterior surface of the iris; also, of eye pigmentation: present only on the posterior surface of the iris; and of a person: having eyes with this form of pigmentation.
• (computing, telecommunications) Of a circuit or device: involving signals which travel in one direction at a time; unidirectional.
• (genetics)

noun

• (algebraic topology, geometry) A generalization of a triangle or tetrahedron to an arbitrary dimension, the generalization being the simplest possible convex polytope for a given dimension; more accurately, the convex hull of n+1 affinely independent points in n-dimensional space.
• (linguistics)
  "The word weak is a simplex. Its derivative weaken is not."
• (linguistics)
  "The question is: is 139. [“Priexal Vanja. ‘arrived Vanya’”] a simplex or is it a cleft structure in which all 'superfluous' constituents were deleted. […] The only indication that 139. is a simplex is the sentence intonation and the absence of a break between the verb and the subject."
• (originally and chiefly US) An apartment (or, sometimes, another type of property) having only one floor or storey; a single-storey property.