1. Immediately: This is the most common meaning. It implies doing something without delay, right now.
* Examples:
* "Please answer the phone at once!"
* "I need you to leave at once!"
* "The doctor arrived at once."
2. Simultaneously: This meaning suggests that two or more things happen at the same time.
* Examples:
* "The music started and the lights went down at once."
* "The doors opened and the crowd rushed in at once."
3. In a single instance: This meaning suggests that something is done in a single, unified action.
* Examples:
* "The company's new policies address both customer satisfaction and employee morale at once."
* "She managed to finish both her homework and her chores at once."
In summary: "At once" generally conveys a sense of immediacy, but it can also express simultaneous occurrence or a single, combined action.