1. in a way that happens naturally without being planned or caused by something else
2. in a way that is done without being forced or pressured by others

Examples

• The crowd spontaneously burst into applause.
• She spontaneously decided to take a different route home.
• The children spontaneously began singing together.
• He spontaneously offered to help with the project.
• The fire started spontaneously due to the heat.
• They spontaneously hugged when they saw each other.
• The idea came to her spontaneously during the meeting.