Othello syndrome
The Othello syndrome is a pscholgogical condition, where a person thinks that their partner is unfaithful, but without having any proof for this behaviour. It is also known as pathological jealousy, morbid jelousy or delusional jealousy.  The person will also behave in ways that are socially unacceptable or abnormal, but which are related to these thoughts. The most common form is characterized by delusions and obsessions. It is considered a subtype of delusional disorder.