In California, there are several different ways you can commit a DUI offense. Under California Vehicle Code Section 23152a, you can be found guilty of being under the influence of alcohol if any amount of alcohol in your system is shown to have debilitated your driving ability below that which would be expected of an ordinary, sober driver.
Under VC Section 23152b, it is a DUI if your blood alcohol content is .08% or higher. For commercial drivers, the standard is a much stricter .04%, and for those under the legal drinking age of 21, any detectable BAC at all (.01% in practical terms) is an underage DUI.
Note that intoxication with drugs so as to affect your driving ability is also DUI, though usually specified as DUID (driving under the influence of drugs.) Any detectable amount of drugs, even prescription or over the counter ones, can be the basis of a DUID if it can reasonably be shown to have affected your driving.
Note that you had to have had the relevant BAC, if a DUI is based on BAC tests, while you were actually driving. A still rising BAC can throw off the results if the test is not taken very soon after the traffic stop. Also note that BAC tests can sometimes be off, and a re-test can be ordered by your attorney that might get a different reading.
Also, if it cannot be proved you were ever driving the car while intoxicated, but you were just found in a parked car or at home with a high BAC, that is not enough to prove DUI. Plus, certain medications, health conditions, low carb or high protein diets, or mouth wash can throw off breathalyzer test results. So it's not true that a .08 reading is the end-all evidence for or definition of a California DUI.