(Commonly, "laptop") A portable personal computer you can carry with one hand. Some laptops run so hot that it would be quite uncomforable to actually use them on your lap for long. The term "notebook" is often used to describe these, though it also implies a low weight (less than 2kg). A "luggable" is one you could carry in one hand but is so heavy you wouldn't want to. One that can by easily operated while held in one hand is a "palmtop".
The computer considered by most historians to be the first true portable computer was the Osborne 1 but see the link below for other contenders.