Consisting Essentially Of
A transitional phrase that limits the scope of a claim to the specified materials or steps and those that do not materially affect the basic and novel characteristics of the claimed invention.
For the purposes of searching for and applying prior art under 35 U.S.C. 102 and 103, absent (without) a clear indication in the specification or claims of what the basic and novel characteristics actually are, "consisting essentially of" will be construed (understood) as equivalent to "comprising.".
Source: U.S. Patent and Trademark Office.