El otoño is another word, like invierno, which disguises the links it has, through Latin, with English. Both autumn and otoño go back to Latin autumnus, but Spanish has changed the shape of the word more, by changing the vowels, and changing the mn to ñ. Notice that you don’t use the article in:
en otoño in the fall, in the autumn
El otoño can also refer to the ‘autumn’ of somebody’s life, their later years. It appears in the title of an early book by the famous Nobel-winning writer Gabriel García Márquez: El otoño del patriarcaThe Autumn of the Patriarch.