Final answer:
A designated port is the one with the lowest path cost to the root bridge, set to a forwarding state to prevent loops in a network.
Step-by-step explanation:
Definition of a designated port is a port that is determined to have the lowest path cost to the root bridge and is placed in a forwarding state for a network segment. This is essential in the operation of the Spanning Tree Protocol (STP), where designated ports are used to ensure that there is only one active path between network segments, preventing loops.