Definition
Using information that one remembers immediately, without prior preparation or research.
"What's the approximate budget for this project?" "I can't tell you off the top of my head, but I can look it up."
"Do you know what topics were discussed in yesterday's meeting?" "I can't recall all the details off the top of my head, but I remember the main points."
"Do you remember when we last met?" "I can't remember off the top of my head, I might need to check my calendar."