The change in pitch or tone of a man's voice when talking to their significant other. The girlfriend voice is characterized by a higher pitch and a more effeminate tone with speech patterns scattered with pet names and childish words.
This type of speech is usually frowned upon when used in the presence of other men. When another man uses this voice they will usually receive a fair amount of ridicule.
"Did you hear Bob's wicked girlfriend voice when he was talking to Lisa? Let's whip his ass!"