It is interesting to me when people say that "search is not a market". So pardon my ignorance but if search isn't a market, why (in the case of antitrust clause

It is interesting to me when people say that "search is not a market". So pardon my ignorance but if search isn't a market, why (in the case of antitrust clauses against Microsoft) are browsers a market? Can somebody explain what constitutes a market?