In quantum field theory, a tadpole is a one-loop Feynman diagram with one external leg, giving a contribution to a one-point correlation function (i.e., the field's vacuum expectation value). One-loop diagrams with a propagator that connects back to its originating vertex are often also referred as tadpoles. For many massless theories, these graphs vanish in dimensional regularization (by dimensional analysis and the absence of any inherent mass scale in the loop integral).
The physics of tadpoles and the word tadpole was invented by Sidney Coleman. The editor was not satisfied, but he changed his mind once Sidney Coleman proposed spermion or lollypop instead. All words are derived from the shape of the Feynman diagram: a circle with a line interval attached to its external side. Tadpole diagrams, in this sense, first appear in a paper by Coleman and Sheldon Lee Glashow in 1964. 
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