Social Security Disability Insurance (SSD or SSDI) is a payroll tax-funded, federal insurance program of the United States government. It is managed by the Social Security Administration and is designed to provide income supplements to people who are physically restricted in their ability to be employed because of a notable disability, usually a physical disability. SSD can be supplied on either a temporary or permanent basis, usually directly correlated to whether the person's disability is temporary or permanent.
Unlike Supplemental Security Income (SSI), SSD does not depend on the income of the disabled individual receiving it. A "legitimately" (i.e. according to the Americans with Disabilities Act, and via other similar legal and medical backing) disabled person of any income level can theoretically receive SSD. Most SSI recipients are below an administratively-mandated income threshold, and indeed these individuals must in fact stay below that threshold to continue receiving SSI; but this is not the case with SSD.
Informal names for SSDI include Disability Insurance Benefits (DIB) and Title II benefits, named for the chapter title of the governing section of the Social Security Act.