What happens if ph meter isn't calibrated. How will it influence ka on a weak acid

I assume you are talking about the calculated Ka arising from using an uncalibrated pH meter? An uncalibrated pH meter has no effect on the Ka as this is a physical constant.

What is the definition of Ka?

If the [H+] was higher or lower how does this affect a calculated Ka, wrt the correct value?

An uncalibrated pH meter might read high or low. If it reads higher than the expected pH, is [H+] higher or lower than expected? how does this higher pH reading affect the calculated Ka?