What is the difference between internal monologue and internal dialog?

Answer 1

Basically one is correct and one isn't. The accepted term in both psychology and literary usage is "internal monologue." A dialogue is between two people. A monologue involves one person.

Although the term internal dialogue is sometimes used by people, it isn't the correct usage, unless of course your characters have multiple personalities.

Answer 2

Internal monologue is the inner voice that narrates one's thoughts and feelings. Internal dialogue involves a conversation within oneself between multiple perspectives or voices.