Final answer:
Secondary union, or secondary intention, refers to the healing process where a wound heals from the inside out when the edges cannot be approximated.
Step-by-step explanation:
Healing where the wound is left open and allowed to heal from the inner layer to the outside surface is known as secondary union or secondary intention.
During the healing process, if a wound is too large or the edges cannot be brought together, it heals by secondary union. This involves the wound edges gradually moving towards each other by a process called wound contraction. Over time, new tissue fills in from the bottom up until the wound is completely closed. This can result in a longer healing time and a more prominent scar compared to the primary union, where the wound edges are brought together and heal much quicker.