Historically, the term “couch” is believed to have derived from the French word “couche,” A couche is a piece of furniture with no arms used for lying down, similar to a chaise. Today, however, the terms couch and sofa are interchangeable. Both refer to a long upholstered seat with a back and arms, designed to seat two or more people.
The term “couch” does have a less formal connotation casual sofa, which is why you’ll sometimes hear “couch” used to describe sofas that are more utilitarian in nature or don’t have a strong design aesthetic. In contrast, those in the design industry will often favor the word “sofa.” All of this said, the terms “sofa” and “couch” can generally be swapped out for each other with no difference in meaning.