Air-integrated dive computers with the capability to download dives to your computer can often provide info. that lets computer software calculate a SAC for your dive, which is handy for learning what your gas use rate tends to run, both on 'easy' dives and those with more stressful conditions.
People often use SAC when another term, RMC (Respiratory Minute Volume) is what they're referring to. As UNCFMP mentioned, SAC is your gas consumption at the surface. You can calculate this by sitting at your computer wearing your scuba mask and breathing off your reg. for a set time (say, 15 to 20 minutes), seeing how many PSI your tank pressure dropped & what volume of gas you used up, then adjusting for a 'per hour' rating.
RMC is actual gas consumption rate on a dive, and factors in issues like depth (faster use at greater depth), conditions (e.g.: swimming into current vs. drift diving), gear (e.g.: dry suit with a of lead, vs. t-shirt & trunks in the tropics) and so forth.
So your dive computer may give you a 'SAC' for your dive, which is actually the Respiratory Minute Volume.
Hope I explained all that without bungling it. Learning about it in the SDI Solo course last year.
Richard.