In the Pokemon games, contact is a property that some moves have when they would realistically make physical contact with the target. If a move is classified to make contact, it activates abilities like Rough Skin or items like the Sticky Barb, which activate when hit by a contact move.
Generally, most contact moves are physical, but contact is classified independent of a move's damage category. Here is more info on contact, as well as a list of moves that are programmed as contact moves.
In relation to Long Reach, the ability reclassifies all of Decidueye's moves as 'non-contact moves', thus Decidueye's moves would not trigger the effects of Rough Skin or the Sticky Barb.
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