Opposite of a smartphone. Generally has lower specs than a smartphone in terms of screen size, wireless connectivity, storage space etc. However the critical differentiator is that there is no development environment that allows any third party applications to be installed (other than java midlets which are very limited). Manufacturers and networks call them 'feature phones'.
This is a near-exhaustive list of smartphone OSes. If a phone doesn't use something on this list it is probably a dumbphone:
* Window Mobile
* Palm OS
All Nokia S40 phones are dumbphones.