An Ultrabook is a mini notebook computer designed to provide similar or superior computing functionality as that of a standard laptop but with less size, weight and an extended battery life.
The Ultrabook is a trademark name that was initially conceived by the Intel Corporation in 2011, but the term has expanded to refer to a new category of notebook computers that sits somewhere between lightweight laptops and tablets. Ultrabooks are integrated with consumer ultra-low voltage (CULV) processors, graphic card accelerators, solid state drives and limited external communication ports.
In general terms, an ultrabook may also be called an ultrapotable notebook or subnotebook.
Techopedia explains Ultrabook
The Ultrabook concept was introduced by Intel as a balanced solution between notebook and tablet computers. The Ultrabook resembles a notebook computer but provides the portability of a tablet PC. Ultrabooks are to be powered by Consumer Ultra-Low Voltage (CULV) Sandy Bridge, Ivy Bridge and Haswell processors, which consume less battery power while providing processing power that's equal to a standard laptop.
The Ultrabook includes a sleek design as compared to its peers and is often mistakenly referred to netbooks, which has low compute power and generally lacks high end features as in an Ultrabook.
Because Intel does not have specifications that specifically define which devices are considered Ultrabooks, the term is less a technology term than a marketing one.