A tenkeyless keyboard is a keyboard without a numeric keypad. Occasionally called a "85%" keyboard.
The term "tenkeyless" is not clearly-defined. The general consensus appears to be that it refers to a keyboard with a fairly standard layout, but with the numeric keypad area removed:
The term has also been used to describe what are now referred to as "75%" keyboards, where the navigation cluster is reduced to a single column and the cursor keys are squeezed into the alphanumeric cluster.
A few (very few) sources also name this layout "85%", abusing the metaphor to relate this layout to the family of compact keyboards.
The numeric keypad is also known as the tenkey, which is something of a misnomer, as the numeric keypad typically contains significantly more than the ten numeral keys 0–9. The numeric keypad is not required by all typists, as it mainly favours those whom requires fast numeric entry and/or dealing with large amounts of numbers. The omission of the numeric keypad is welcomed by many, as the removal of over 20 redundant keys from the keyboard reduces the keyboard's footprint, allowing right-handed users to keep the the mouse or trackball within easier reach.