Disability is often misunderstood. For instance, it is not a "handicap." Living with a disability means that you have a condition that was caused by an accident, trauma, genetics, or disease that may limit your mobility, hearing, vision, speech, or mental function. Anyone can experience a disability and probably will if they live long enough. That said, many of the agencies with which veterans will engage define disability differently.
a. The Department of Veterans Affairs
The Department of Veterans Affairs (VA) assigns disability as a percentage (anywhere from 0% to 100%). To qualify for VA disability compensation, you must have a disability that VA determines was incurred or aggravated as the result of your military service, and thus deemed to be "service connected." VA often uses service connection to determine entitlements to other benefits including health care and vocational rehabilitation.
VA disability ratings are not arbitrarily assigned. A medical examination must be completed in order to evaluate and assess the severity of disability before a rating can be determined based on the VA's Disability Rating Schedule, which is organized under a variety of body systems, including cardiovascular system, digestive system, respiratory system, and mental disorders. The Rating Schedule correlates injury and/or illness to a percentage rating that estimates the reduction in average earnings capacity caused by the disability.
Resources for more information about disability compensation:
b. Social Security
Social Security has a different definition of disability. Social Security does not use percentages of disability. You are either determined to have a disability or you do not. To be found disabled: (1) you must be unable to do substantial work because of your medical condition(s); and (2) your medical condition(s) must have lasted, or be expected to last, at least one year or to result in death. Military servicemembers may be eligible to receive expedited processing of disability claims from Social Security. Benefits available through Social Security are different than those from VA and require a separate application. You can receive Social Security Disability Insurance and VA disability compensation benefits at the same time. Eligibility under one system does not mean entitlement under the other.
More information about Social Security Benefits for Disabled Veterans:
c. Employment and Education
Defining disability in employment and education is much more complicated – and is defined under the law to ensure equal protections and nondiscrimination. It is defined by type, category (cognitive, emotional, physical, etc.) or diagnosis (bi-polar disorder, learning disability, quadriplegia, etc.), and may often be defined by personal experience. To make matters even more confusing, being considered a person with a disability in one workforce or academic context does not necessarily equate to being a person with a disability in another. More about this topic can be found in the section, Disability in the Eyes of the Law.
 Service-connected disabilities are defined as current chronic disabilities diagnosed by a medical professional and determined by the VA to have been caused or aggravated by military service or secondary to an existing service-connected disability.