The Collaborative International Dictionary of English v.0.48:

Seventeen \Sev"en*teen`\, a. [OE. seventene, AS. seofont[=y]ne,
   i. e., seven-ten. Cf. Seventy.]
   One more than sixteen; ten and seven added; as, seventeen
   years.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Seventeen \Sev"en*teen`\, n.
   1. The number greater by one than sixteen; the sum of ten and
      seven; seventeen units or objects.
      [1913 Webster]

   2. A symbol denoting seventeen units, as 17, or xvii.
      [1913 Webster]

WordNet (r) 3.0 (2006):

seventeen
    adj 1: being one more than sixteen [syn: seventeen, 17,
           xvii]
    n 1: the cardinal number that is the sum of sixteen and one
         [syn: seventeen, 17, XVII]