A criminal defence lawyer appointed within criminal justice proceedings in the USA, paid for by the state, but representing an indigent defendant in a criminal matter.
To have access to a public defender, a criminal defendent would normally be required to take a pauper's oath.
The Los Angeles Office of the Public Defender (pd.co.la.ca.us) describes public defenders as:
"... licensed attorneys who devote their full time to representing persons who have been accused of a crime, have no attorney, and lack the means to employ one. Others who appear in the juvenile and mental health courts are likewise represented.
"There is no distinction between the obligations of a defender toward his client and the duties of a private attorney in the same circumstances."
Would be known as legal aid or "aide juridique" lawyer in Canada.