Deadname is a term that functions as both a noun and a verb. As a noun, It refers to the birth name of a person who has since changed their name. As a verb, it refers to the practice of calling someone who has changed their name by their birth name. The term can be applied to anyone who has changed their name but is most often attributed to trans people.
More About Deadname
While referring to someone by their deadname (their birth name or terminated name) is problematic, in the case of trans individuals whose changed name is typically reflective of their gender, using a deadname (or "deadnaming") is particularly disrespectful as it effectively mis-genders them. To avoid this, it is always best to refer to anyone by the name they ask you to use.
For many, the issue of a deadname can come up with government organizations or internet accounts, as in Facebook's infamous "real name" policy.