A handheld computer, also called a palmtop computer, is a term that has variously been used to describe a small-sized personal computer (PC) typically built around a clamshell form factor and a full keyboard, including: Palmtop PCs, personal digital assistants (PDA), ultra-mobile PCs (UMPC) or portable gaming PCs. The brand Handheld PC specifically is a now-defunct class of computers introduced in the 1990s that was marketed by Microsoft, and is detailed below.