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The practice in which a broker conducts a transaction on his/her own account after filling a similar order on behalf of a client. For example, if a client sells 10,000 shares and the broker owns some shares in the same company, he may piggyback by selling his own shares. A broker piggybacks when he/she believes that the client has insider information, or at least a better understanding than the broker on the market's future movements. Piggybacking should not be confused with piggy back registration, which is a different concept altogether.