It is a symphonic genre, not so much in musical form (in which it is more similar to the organ sonata or suite), but in imitating orchestral tone color, texture, and symphonic process.
Though the very first organ symphony was written by German composer Wilhelm Valentin Volckmar in 1867, the genre is mainly associated with French romanticism.
César Franck wrote what is considered to be the first French organ symphony in his Grande Pièce Symphonique several years earlier, and the composers Charles-Marie Widor, who wrote ten organ symphonies, and his pupil Louis Vierne, who wrote six, continued to cultivate the genre.
Modern composers such as Jean Guillou have written organ symphonies as well.
3 and the Symphony for Organ and Orchestra by Aaron Copland, though strictly speaking such pieces are closer in form to orchestral symphonies than to the solo organ works described above.