A presheaf is a mathematical structure used in category theory that assigns data to open sets of a topological space in a way that respects the inclusion of open sets. It generalizes the notion of functions or sections defined on open sets and is foundational in the study of sheaves, which impose additional conditions of locality and gluing.
An open set in a topological space is a fundamental concept used to define continuity, convergence, and connectedness. It is a set where, for every point within it, there exists a neighborhood entirely contained in the set, enabling the formulation of limits and continuity without relying on a specific metric.