Unfortunately, time in this instance has reduced the distinction
between a sofa and a couch so that today the two terms are largely
interchangeable. According to our friends at Collins
Dictionary, a sofa is "an upholstered seat with back and arms for
two or more people" and a couch is "a piece of upholstered
furniture, usually having a back and armrests, for seating more
than one person. It is also a bed, especially one used in the
daytime and by the patients of a doctor or a psychoanalyst."
If you drag out your history books, the differentiation becomes
a tad clearer. In simplistic terms, a couch is a piece of
furniture that you recline on whereas a sofa is for two or more
people to sit on.
To quote the historians, a couch "can be used as a sofa by day
and a bed by night"; "in the 17th and 18th century a couch
was usually an armless chair with a greatly elongated seat with a
slanted and sometimes hinged back used for reclining or sleeping
during the day"; "in the late middle ages the couch signified a
daybed".
Given that the term couch also means to "lay down" and that the
couch has strong links to doctors and psychologists rooms, the
distinction seems clearer - it would appear that whoever coined the
phrase "couch potato" had a strong appreciation of history.
Sofa originates from the dias on which a Grand Vizier (Muslim
high official) sat in the 17th century. The sofa evolved from
the upholstered armchair. Today the term sofa is largely
synonymous with settee, despite the settee beginning life as a
double chair furnished with cushions. Into the 19th century,
the sofa was often thought of as a ladies' lounging seat furnishing
the drawing rooms of wealthy homes. It was not until the
industrial revolution that sofas began to appear in the homes of
the general populous.