F-spacing = FENDER spacing, the measurement reference for Fender whammy guitar bridges which is a little wider than others in terms of string spacing.
it's also the same measurement for Floyd Rose bridges & its derivatives (eg: Ibanez's Edge bridge) so when you buy replacement pickups, there would be indicators for this reference for instance, DiMarzio has the F-spaced models, Seymour Duncan has the Trembucker models.