The process of bringing an immigration-related lawsuit to federal court. Such a lawsuit makes a claim that the government has violated a particular statute, regulation, or constitutional provision.
An immigration-related complaint can be filed in federal court to challenge:
Orders of removal on legal or constitutional grounds (discretionary cannot be challenged in federal courts)
“Release from custody” decisions:

Unlawful detention of someone in immigration custody due to DHS’s refusal to set bond, or the conditions of release
Legalization denials/Legal Immigration Family Equity Act of 2000 (LIFE Act) cases
Naturalization decisions:

USCIS’s denial of an application for naturalization
“Delay cases”:

USCIS’s unreasonable delay in adjudicating an application or petition
Other agency decisions:

Non-removal and non-detention decisions subject to review, such as denials of petitions forvarious immigration classifications; denials
of applications for change/extension of status; denials of applications for employment authorization; etc.