A nexus letter is the common name for a written medical opinion on whether a veteran's current condition is linked to their military service. In it, a clinician who has reviewed the veteran's history states a professional opinion on the connection, usually in the probabilistic language VA adjudication recognizes, and explains the clinical reasoning behind it.

The letter is one form of evidence bearing on one element of a claim: the connection to service. It does not establish the diagnosis or document severity; those live in the broader medical record. And like any medical opinion, it carries weight only to the extent it is clinically supportable: grounded in the record, reasoned, and signed by a clinician exercising independent judgment. An opinion written to order, unsupported by the underlying records, is weak evidence wherever it comes from.

The plain-language name for what the letter addresses is connection to service, and that is the more useful term to think in: it names the element of the claim rather than a document, and it keeps the focus on the question any opinion has to answer from the clinical record.