In 2013, the ICANN Board's New gTLD Program committee defined a "Closed Generic" as a "TLD string that is a generic term and is proposed to be operated by a participant exclusively for its own benefit."
The term "closed" refers to a gTLD wherein registration is limited exclusively to the registry operator and its affiliates (See Specification 11, Section 3(d) of the gTLD Registry Agreement).
A definition of a "generic" string can be found in ICANN's gTLD Registry Agreement: "A string consisting of a word or term that denominates or describes a general class of goods, services, groups, organizations or things, as opposed to distinguishing a specific brand of goods, services, groups, organizations or things from those of others." (See Specification 11, Section 3 (d)).
In January 2024, the ICANN Board determined that applications for Closed Generic applications "will not be permitted until such time as there is an approved methodology and criteria to evaluate whether or not a proposed closed domain is in the public interest."
Currently, ICANN is accepting self-assertion as part of the application process that the applicant does not intend to operate a gTLD as a "Closed Generic". If ICANN receives comments regarding a string (e.g., via Application Comment Forum) and concerns regarding "genericness," ICANN would review the comments and string at that time.