A "veto letter" is a written document that is used to reject or deny a proposed idea, decision, or action, usually by someone with the authority to do so. It is like saying "no" to a suggestion or plan. Full definition
See this story, which contains the Governor's veto letters (scroll down to find HB 533). (thirdpartypolitics.us)
In his 2015 veto letter, Cuomo wrote that the bill «would allow attorneys» fees to be assessed against a state agency, even if the state agency ultimately prevails.» (gothamgazette.com)
«The proposed ordinance will authorize a contract that could be used by the CRB's executive director to expand the CRB beyond its current role, which is to review citizen complaints and provide a recommendation to the chief of police, into assisting complainants with civil litigation,» Miner wrote in her veto letter. (syracuse.com)