Topology is a system of sets, which is closed under arbitrary unions and finite intersections.
Is there a name for lattice, which has analogous properties, i.e., every subset has supremum. (And also every finite subset has infimum, but this is already in the definition of lattice.
A lattice with suprema for all subsets is called a complete lattice, or perhaps a cocomplete lattice if you are category-theoretically inclined. Here's a fun fact: any lattice that has suprema for all subsets also has infima for all subsets. The infimum of a subset is, of course, the supremum of all the lower bounds for that subset.
Unfortunately, a homomorphism of lattices need not preserve infinite suprema/infima, and here is where the subtlety is: for example, when one says "(co)complete join semilattice", one is thinking of homomorphisms that preserves all suprema, even though a (co)complete join semilattice is automatically a complete lattice. A continuous map of topological spaces induces a homomorphism on the lattice of open sets that preserves finite meets and infinite joins, which is precisely the definition of a homomorphism of frames. (A frame is a complete lattice that satisfies the infinite distributive law.)