The Wildcat (Default) Skin is a Common Fortnite Outfit.
Wildcat is the unofficial name of this default skin you can potentially use when not selecting a skin in Fortnite: Battle Royale. I say unofficial because that name is used in the Save the World mode, but is never really referenced in the battle royale. Default Skins are generally referred to as "no skins" due to the fact that you aren't wearing a skin. This is usually in a negative connotation and describes players that are new to the game because they don't have a skin. However, there's also now "fake no skins" that are really good players at the game and purposely don't wear a skin to make other players underestimate them. TFue in particular has popularized this line of play.
Wildcat was removed as a default at the start of Chapter 2: Season 1.
When Epic creates certain skins they use some of these default skin templates and change their clothes and other features. I've included some of them below, but it's not meant to be an exhaustive list.
Wildcat (Default) Styles & Images
You can find additional Wildcat (Default) styles and alternative images below!