A "situationship" is a relationship, usually romantic or parental in some way, that is very , very problematic. Whether it be that one partner is an abuser, hate each other, or even one partner is only in the relationship for personal gain (or all of the above), situationships never end well.

Romantic examples:
Natz and Estelle Ivanov (see Natz's Affair)
Lee and Victoria Ainsworth
Antoinette and S
X and Vi (It's complicated)

Non-Romantic examples:
Vanessa and Soren (see The Mariposa Scandal)
Antoinette and Loki
Jace and Azazel