what is the meaning of CPM ?

'Cost-Per-Impression' is also known as 'CPM'
It is the term used for calculating the cost of each click in PPC campaigns.

Cost per impression is an online advertising which is related to web traffic and refers to the cost of internet marketing campaigns where advertisers pay for every time an ad is displayed. It is derived from advertising cost and the number of impressions.

CPM model refers to advertising bought on the basis of impression. This is in contrast to the various types of pay-for-performance advertising, whereby payment is only triggered by a mutually agreed upon activity.

CPM is an acronym for Cost-Per-Thousand. Many online ads are priced on a CPM basis in which the cost is expressed for 1000 impressions.