あんざん暗算anzan
1.
The act of performing calculations in one's mind without the use of written work or calculation tools.
2.
A method of deriving numerical answers solely through mental processes, bypassing traditional calculation methods such as written arithmetic or using abacuses.
3.
The skill of executing mathematical operations entirely in one's head, without external aids or references.
Example:
彼は暗算をするほど頭が良くない。