A bid is a price that marketers are willing to pay for displaying their advertisement.
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A bid is used in PPC, or pay-per-click advertising, and is often referred to as keyword bidding, which is the sum an advertiser is willing to place on a particular keyword so that Google considers them in their algorithm and place them at the top of their search results or ads. Businesses online bud on specific keywords in an attempt to effectively secure ad space for important keywords relevant to their business.