A region in the variable domain of a protein which belongs to the immunoglobulin superfamily, and which is less "variable" than the CDR

Complementarity determining region

Complementarity determining regions are regions within antibodies or T cell receptors where these proteins complement an antigen's shape. Thus, CDRs determine the protein's affinity and specificity for specific antigens...

.
As an example, each chain (alpha or beta) of the TCR

T cell receptor

The T cell receptor or TCR is a molecule found on the surface of T lymphocytes that is responsible for recognizing antigens bound to major histocompatibility complex molecules...

 is composed of 4 framework regions, separated by 3 CDR

Complementarity determining region

Complementarity determining regions are regions within antibodies or T cell receptors where these proteins complement an antigen's shape. Thus, CDRs determine the protein's affinity and specificity for specific antigens...

.

The nomenclature "framework" region comes from the fact that they would roll back at the moment of recognition of the antigen

Antigen

An antigen is a foreign molecule that, when introduced into the body, triggers the production of an antibody by the immune system. The immune system will then kill or neutralize the antigen that is recognized as a foreign and potentially harmful invader. These invaders can be molecules such as...

, making the 3 CDR

Complementarity determining region

Complementarity determining regions are regions within antibodies or T cell receptors where these proteins complement an antigen's shape. Thus, CDRs determine the protein's affinity and specificity for specific antigens...

closer to each other.