The term toxic relationship is used to describe a relationship where the couple is destructive towards each other rather than constructive and supportive. The destruction or toxicity is typically coming from one person, but it is possible for both people to become competitive and unsupportive of each other. In these cases, one person initiated the toxic behavior, and the other person responded in kind. In extreme cases, the toxicity can include physical abuse, but it is emotionally damaging even at best for the partner.
A toxic person or partner does not start that way. These people are manipulative, and they recognize that putting on a mask and being a positive, supportive person is important. However, even with their best attempts at covering up the toxicity, there are often subtle signs of potential problems.