a (alpha, also a-level)

When the null hypothesis is true and you reject it, you make a type I error. The probability of making a type I error is called alpha (a) and is sometimes referred to as the level of significance.

When you are determining the a for your test, you need to consider the seriousness of detecting a significant association when one does not really exist. The more serious the error, the less often you will be willing to allow it to occur. Therefore, you should assign smaller probability values to more serious errors.