De-embedding is a post-measurement process to minimize errors and reveal information about the device under test. When there is a composite measurement of a device under test or fixture combination, you can use de-embedding to isolate the performance of the fixture and extract (de-embed) the fixture from the measurements. 

De-embedding is mathematically removing the measurements affected by the fixture leaving only the behavior of the device under test. This is commonly used when there are non-coaxial connection from the VNA to the DUT, and it is used on circuit board traces, backplane channels, semiconductor packages, connectors or other discrete components.