simplex

Having a single structure; not composite or complex; undivided, unitary.

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.

Of a circuit or device: involving signals which travel in one direction at a time; unidirectional.

Of a polyploid organism: having one dominant allele at a given locus on all homologous chromosomes.

Synonym of heterozygous (“of an organism: having two different alleles in a given gene”).

Of a word: having no (derivational) affixes; simple, monomorphemic, uncompounded.

Of an apartment (or, sometimes, another type of property): having only one floor or storey; single-storey.