Capacitor Selection for LM2931 Voltage Regulator

Hey everyone, I'm working on a circuit that's designed to be used in a car, and powered at all times. I'm going to be using an AtTiny84 with a few push buttons and a few transistors hooked up to it. My best guess is that the whole circuit shouldn't ever draw more than 50mA, and that's probably an overestimate. The problem I'm having is determining which capacitors (type and rating) to use for this voltage regulator (data sheet). In the data sheet, it says that the input capacitor should be .1uF, and that the "magnitude of the output capacitor impedance |ZO| must not exceed 0.4 ohms" (page 8). The problem is, I don't understand the capacitor impedance rating, and I can't find anything that indicates it on the data sheet for the capacitors I was looking at. I've been doing some reading up on capacitors, but I'm having trouble trying to figure out which capacitors will work for this application, and if it has anything to do with ESR, a term that keeps coming up. Thanks!

Yes ESR means effective serial resistance and is a prameter of the capacitor, it should be in the data sheet, and it is that value that has to be less than 0.4R.

Thanks! After your reply and some more research, I think I may have a better picture of what I'm looking at now. I was confused by the mOhm, thinking that it was megaOhms and that I must be missing something. I see now that it's milliOhms. I read a Texas Instrument note that talked about how ceramics are usually bad for LDO linear regulators due to their low ESR, however, the data sheet for this voltage regulator doesn't appear to have a lower limit for ESR, so a ceramic should work, right? I was thinking a 22uF multilayer ceramic rated at 10v for the output, and a 0.1uF multilayer ceramic rated at 100v for the input. Does this sound right, or do I need to use a tantalum or electrolytic capacitor with a slightly higher ESR?

I was thinking a 22uF multilayer ceramic rated at 10v for the output

That will probably work, but when you get to 1uF or more anything other than an electrolytic or tantalum can get expensive and bulky.

The data sheet recommends a tantalum or a good quality electrolytic with a wide temperature range.

DVDdoug:
That will probably work, but when you get to 1uF or more anything other than an electrolytic or tantalum can get expensive and bulky.

The data sheet recommends a tantalum or a good quality electrolytic with a wide temperature range.

I did look at the tantalum and electrolytic (by that I mean aluminum electrolytic, is there another type I should be paying attention to?), but they were all really pricy for ones with a low enough ESR (less than .4ohms). I searched through Digikey and Mouser, but there is a good chance I'm missing something as this is my first time trying to pick out capacitors for my own circuit.

Edit: I did some more research, and I looked at aluminum electrolytic caps again and at some aluminum organic polymer capacitors. I was clearly missing something before, probably getting hung up on ESR values. Either way, I found many capacitors that will work at decent prices. thanks to Grumpy_Mike and DVDdoug I think I can pick some caps that should work perfect. Thanks guys!