A person who helps communicate between speakers of different languages. Or, one who explains dreams.
Joseph pretended to need an interpreter to speak to his brothers (Genesis 42:23). Also, dreams needed to be interpreted (Genesis 40:8; 41:15–16; Daniel 2; 4:6–9, 18–24; 5:7–8, 12–17; 7:16). The interpreter was sometimes one who acted as a mediator (Job 33:23).
Ezra and Nehemiah acted as interpreters or translators of the law of Moses when it was read to the Jews who had returned from exile (Nehemiah 8:8–9). They must not have known Hebrew.
In New Testament times, the interpreter:
Explained the words of those who spoke in tongues (1 Corinthians 14:28)
Translated foreign languages (Acts 2:6)
Explained the Scriptures (Luke 24:27)