# Definition:Underlying Set

## Definition

### Abstract Algebra

Let $\struct {S, \circ}$ be an algebraic structure.

Then the underlying set of $\struct {S, \circ}$ is the set $S$.

### Relational Structure

Let $\struct {S, \RR}$ be a relational structure.

Then the underlying set of $\struct {S, \RR}$ is the set $S$.

### Metric Space

Let $M = \struct {A, d}$ be a metric space.

Then the underlying set of $M$ is the set $A$.

### Topological Space

Let $T = \struct {S, \tau}$ be a topological space.

Then the underlying set of $T$ is the set $S$.