The Wolfram Language's units system includes thousands of different quantities, including units, physical constants, and IndependentUnit expressions. Supported units include all those specified by NIST Special Publication 811.
Many quantities have multiple common names, which can make them difficult to find in a long list of possible units. The Wolfram Language's built-in unit interpretation system allows you to specify units and physical constants using natural language input.
One method of unit discovery in the Wolfram Language is the WolframAlpha function and the related free-form input methods it provides.
Quantity will also automatically try and interpret unrecognized units. Once a unit has been interpreted, it will be cached so that subsequent evaluations will use the same result.
Unit interpretation requires internet connectivity, and can entail additional evaluation time. If speed is a concern, it is advisable to use the canonical unit specification, which can be found using InputForm.