This term originally distinguished the featured film, the film that
drew the audience to the theater, from the
B picture.
Back in the day, American cinemas exhibited films in a significantly different way than they do today. The program might have included cartoons, short films, a newsreel, the featured presentation followed by previews of coming attractions (hence the term trailer) and then a second film for those who wished to stick around. The featured film was the big, slick Hollywood production. The second film, the B picture, was usually low-budget and often of one of the exploitation, science fiction or horror genres.
The economics of film distribution have changed radically since then. Today the strategy requires fitting in the maximum number of screenings of each film being shown. As such, exhibitor chains and distributors try very hard to keep the run time of films down below 120 minutes.
With the discarding of cartoons, shorts, newsreels and B pictures, all that's left is the feature.
Today, the term "feature film" is primarily an indication of duration. Most films considered to be features are at least 90 minutes long. Most film festivals define anything longer than 45 minutes as a feature film.