inertial frame ?

[gandharva_23]

1. The problem statement, all variables and given/known data
How can i define a perfectly inertial frame ?

[CompuChip]

You can't.
The earth goes around her axis, which goes around the sun, which goes around the galaxy, which goes around ....

Just take a frame that is "big enough" (e.g. if you're considering atoms, the earth will usually be big enough, but if you're considering oceanic flows, you need to take the earths rotation into account) - if you do that carefully it will count as inertial.