Abbreviated as SOV, share of voice is defined as the share of total advertising exposures that a brand gets. Traditionally, calculating share of voice involves determining how big your share of advertising is compared to all of your competitors.
In a nutshell, it is a way to measure a brand’s presence and to gauge a brand's visibility within an advertising medium during a specific time period. It is usually represented as a percentage of the total impressions.
SOV is usually calculated on an ad unit basis, and can be referenced as a percentage of a single ad unit on a page or group of pages. For example, assume that every page on a site contains a 728×90 leaderboard and the site generated about 10 million pageviews monthly. A proposal for 1 million leaderboard impressions during a one month period would equate to a 10% share of voice.