Frontside
Frontside and backside are surfing, skateboarding, snowboarding, scootering, BMX bikes and aggressive inline skating terms that are used to describe how a person approaches an obstacle or performs a certain trick. In Aggressive Skating, frontside and backside are types of grinds.
The names frontside and backside originate from surfing where they mean the direction the surfer is facing while surfing a wave. If the surfer is facing the wave, he or she is surfing frontside, otherwise he or she is surfing backside. The terms forehand and backhand are synonyms for frontside and backside but they are only used in surfing.
Skateboarding
For performing rotational tricks on flat ground, frontside means that for the first 90-180 degrees of the rotation, the rider rotates to face the direction of travel. For a rider traveling in the "regular" stance (left foot leading), frontside means to rotate counter-clockwise (as seen from above). Conversely, a skater doing a backside rotation/trick is rotating clockwise. The same applies to horizontal board-rotation tricks, where the skater does not rotate: a regular stance skater performing a frontside shove-it spins the board anti-clockwise, as seen from above.
When riding fakie/switch, the term is reversed. For example, a rider in the "goofy" stance (right foot leading) does a frontside 180 when he/she rotates 180 degrees clockwise.
When applied to tricks involving obstacles, backside and frontside take on different meaning: the terms define how the obstacle is being approached. For example, when performing a frontside boardslide, "frontside" means that the obstacle (e.g. rail or ledge) is to the front (the toe-side) of the skateboarder. To carry out a frontside boardslide, a regular-stance skater will rotate slightly clockwise before sliding, facing away from the direction of travel. This is the opposite direction of rotation to a frontside ollie.
Snowboarding
In snowboarding, frontside and backside have the same meanings as in skateboarding. When turning, backside is analogous to a heelside turn, and frontside is analogous to a toeside turn.
In the air, backside means that you turn the front of your body into the rotation first and frontside means you turn your back into the rotation first. For example, a left foot forward (or regular) rider would rotate counterclockwise to do a frontside 360 and clockwise to do a backside 360.
Aggressive Inline Skating
In Aggressive Inline Skating, frontside and backside are used to describe the way the skater's feet are positioned when grinding on an object. For example, if a skater jumps on to grind a ledge with both feet landing on the h-blocks (the grindable area in the middle of the frame), with the toes facing the ledge, the skater is doing a frontside grind. Their whole body will be facing the object.
A backside is much the same, although when the skater jumps on, they will be landing with their back to the object, and with their heels facing the ledge.