A function taking values in a metric space is bounded if its image is a bounded set.
The underlying idea is the following: a bounded set can be covered by a single ball of some radius.
For example, the left or right order topology on a bounded set provides an example of a compact space that is not Hausdorff.
Furthermore, any bounded set in H is weakly compact.
A bounded set would describe a group with clear "in" and "out" definitions of membership.
The closure of a bounded set is bounded.
The polar of a bounded set is an absolutely convex and absorbing set.
Note that this definition does not require that the domain of the given function is a bounded set.
However, in any case a bounded set can be specified which contains the actual value of D.
Image of any bounded set under T is relatively compact in Y.