In boardsports, fakie is riding backwards with the tail facing in the direction of travel. When used in conjunction with a trick name, like "fakie ollie", it means that the trick was performed as it would normally be done with the exception of riding backwards. Not to be confused with "switch" or switchstance which is literally "switching" ones stance. (from either regular or goofy to the opposite.) with the nose pointing in the direction of travel.
In skateboarding, people will add "to fakie" at the end of the name if the rider does a revert or lands a slide in their switch stance when they went into the trick in their normal stance. (e.g. frontside boardslide to fakie, backside tailslide to fakie, etc.)
In snowboarding, in a park event, landing the other way from natural is landing switch. In a pipe event, It is called landing fakie. Freeride, powder, and alpine snowboards are non-symmetrical so theoretically riding them backwards with the tail facing downhill is riding fakie, and riding with switched bindings (goofy setting if you are regular) with the nose pointing downhill is riding switch.
In BMX a fakie is riding backwards and pedaling with the sprocket movement as you are moving backwards then at the right point turning your bars the opposite direction you want to turn and sliding out so you are riding forward again.
In Freestyle Skiing the act of descending the slope backwards is referred to as "switch" rather than "fakie".