tercerista
third-party intervener
tercero
third
+
-ista
person who does something
Tercerista means third-party intervener and can be thought of as tercero0 (third0) + -ista1 (person who does something1), literally meaning 'a person who is third' or intervenes as a third party.
noun
- third-party intervenerA person who intervenes as a third party in a legal dispute or conflict.El tercerista presentó una demanda en el juicio. // The third-party intervener filed a claim in the lawsuit.