Simply put, a toxic relationship is any relationship that is perceived through a person in the relationship in question as toxic.
For example, if someone says, "I just got out of a toxic relationship," that is a broad statement that could mean a lot of different things. That person could mean that they were getting neglected, beaten, cheated on, lied to, taken advantage of, or anything negative that would have an impact on that person physically, mentally, or emotionally.
Toxic relationships should not be taken lightly. They're very real and very dangerous. They are called "toxic" for a reason. Toxicity spreads, and if a person is in a toxic relationship, that toxicity can spread to their way of thinking and how they live their life. Mental conditions such as anxiety and depression can be an onset.
I want to be clear that some toxic relationships are not as dire as others. Where someone could be getting beaten in their relationship, someone else could simply be suffering from being ignored or lied to. Both are equally as toxic, but the situation where the person is being physically abused is a more dire situation.