BT BB 27b(34) :מאן תנא דבנזקין בתר אומדנא דהשתא אזלינן‏ : who is the tanna (who holds) that as regards preventing damages we follow only the current assumption (as opposed to assumed future changes in the conditions)?
See the entire entry for ˀumdān, ˀumdānā n.m. estimate