1. n. 
The resistivity of a formation measured by flowing current in a horizontal plane. In anisotropic formations the horizontal and vertical resistivities are different. In a vertical well, wireline induction logs and measurements-while-drilling propagation logs measure the horizontal resistivity, whereas laterologs measure the horizontal resistivity with some component of the vertical. In deviated and horizontal wells, all these logs measure some mixture of both vertical and horizontal resistivity.