Dictionary.net

Back-sight. In levelling along a line, suppose the staff to be held at points A, B, C, D, etc., successively, the level is first placed between A and B, then between B and C, then between C and D, and so on; in these positions the surveyor looks back to A, B, C, etc., and forward to B, C, D, etc., and in each case reads the staff; the former readings are called back-sights, the latter fore-sights.