The term friction loss (or frictional loss) has a number of different meanings, depending on its context.  
* In fluid flow it is the head loss that occurs in a containment such as a pipe or duct due to the effect of the fluid's viscosity near the surface of the containment.
* In mechanical systems such as internal combustion engines, the term refers to the power lost in overcoming the friction between two moving surfaces.
* In economics, frictional loss is natural and irrecoverable loss in a transaction or the cost(s) of doing business too small to account for.  Contrast with tret in shipping, which made a general allowance for otherwise unaccounted for factors.