Final answer:
A dyad refers to a social group with two members. If one member leaves, the dyad ceases to exist, unlike a triad, which consists of three members and can survive the departure of one.
Step-by-step explanation:
The concept that refers to a social group with only two members is known as a dyad. A dyad is characterized by having a very simple social structure and direct interaction between its two members. If one member leaves the group, the dyad ceases to exist. In contrast, a triad is a social group consisting of three members, which allows for more complex relationship dynamics, including the potential for majority opinions and the ability to survive the withdrawal of one member.
In a dyad, if one person withdraws, the group can no longer exist, whereas in a triad, the group can continue even if one person leaves.