A portmanteau of "polyamory" or other relationship models beginning with "poly" and "molecule". Polycule is used to describe any system of connected non-monogamous relationships.
A polycule can be as simple as a "V" (A is dating both B and C, with their approval, but B and C are not dating each other) or a triangle (A, B, and C are all dating each other), but it can also get very complex. (Ex: A is dating B and C. B is dating A and D. C is dating A, E, and F. F is also dating G.)
When drawn out as a chart, these connections can look similar to a molecular structure- hence the name polycule.
"My boyfriend's wife just got a new girlfriend. Looks like I'll have to update our polycule!"
"My OTP is Malcolm, Sheila, and Gary. ... What? One True Pairing? No, I meant One True Polycule."