underflow Syllabification: (un·der·flow)Pronunciation: /ˈəndərˌflō/Definition of underflow noun 1an undercurrent. a horizontal flow of water through the ground, especially one underneath a riverbed. 2 Computing the generation of a number that is too small to be represented in the device meant to store it. underflow in other Oxford dictionaries Definition of underflow in the British & World English dictionary