Instead of "Overture" Wagner uses the term: Einleitung (introduction). Maybe because Wagner composed at that time an "introduction" for each of the three acts of his operas. 

The composition of Tristan und Isolde was finished in 1859. The Prelude was premiered in 1860, the complete work in 1865. See score example x (LINK TO CORRECT LATER  in section 6. 

Some interesting info about Tristan und Isolde you can for instance find here (German wiki).