done with a focus on speed of calculation rather than accuracy. This change in priority often results in wild approximations and unvalidated assumptions, leading to low quality answers. Often used in rough-work to check previously calculated answers, or in physics
See also, napkinmath
Red: Quick! How much worse would it be for Venus
to crash into the Earth
*pause for quickmath*
Blue: Well, Venus has twice the radius, and it's a volume, so... 8. It would be 8 times as bad.
Red: Wow. Science