Avg. CPC is defined as the total cost incurred for total clicks. Avg. CPC defines the the actual amount you are charged for each click. Avg. CPC = Total Cost/Total Clicks
If a user clicks on your ad, then google ads counts that as one click.
When your ad gets displayed on the Search Engine Results Page (SERP) it is called an impression. Impression is counted each time your ads show on Google search engine result page or any other site on the Google network.