Given a ring, or a $k$-algebra (unital or not) $A$, its maximal spectrum $Spec_m A$ is the set of its maximal ideals.
If $k$ is a field, and $R$ is a finitely generated noetherian commutative unital $k$-algebra without nilpotent elements, then $Spec_m A$ equipped with the Zariski topology is a noetherian topological space; the varieties in the classical sense (cf. chapter 1 of Hartshorne) are exactly the spectra of such $k$-algebras. A more appropriate spectrum for general commutative unital rings is the prime spectrum.
In functional analysis, there is a slight variant of this notion, defined using (automatically continuous) characters, the Gel'fand spectrum of a $C^*$-algebra, where however the topology is much richer, indeed compact Hausdorff (locally compact Hausdorff, in the non-unital case).
In analytic geometry one also uses analytic spectra.