Final answer:
Vygotsky defined private speech as the self-directed speech that children engage in while thinking and solving problems. It serves as a tool for children to regulate their own thinking and behavior.
Step-by-step explanation:
Vygotsky defined private speech as the self-directed speech that children engage in while thinking and solving problems. This speech is typically spoken out loud but is not intended for communication with others. According to Vygotsky, private speech serves as a tool for children to regulate their own thinking and behavior. For example, a child might talk to themselves while working through a math problem, using language to guide their thought process.