Why is it called search arbitrage?
Arbitrage means buying something in one market and selling it for more in another. In search arbitrage the thing being traded is a visitor. Clicks are bought where attention is cheap, such as a social feed, and resold where intent is valuable, on a page of search ads. "Search" refers to the search-ad feed that supplies the revenue.
In finance, true arbitrage is close to risk-free because both prices are known at the same moment. Search arbitrage is not like that. The buying price (CPC) is known immediately, but the selling price (RPC) is only an estimate until the search engine finalises revenue weeks later. So it is better described as a trading business with real risk than as a guaranteed gap.
The same idea applied to other ad formats is called traffic arbitrage or ad arbitrage. Search is the popular version because someone who has just chosen a query has shown commercial intent, and advertisers pay far more for that than for a passive banner view.