Selfies are a style of photography wherein
1) the photographer's own face is included in the photograph,
2) and the camera is held by the photographer when the photo is taken.
Other optional elements are often associated with selfies. Use of a mirror is common, especially mirrors found in dirty, unphotogenic bathrooms. Bathrooms are a popular location for selfies since so many believe that the worse everything around them looks in the photo, the better they will appear by comparison.
Immediate upload of the photo to social media is also optional, but generally not recommended. By reviewing the photo on a large computer monitor one has a better chance to find and remove unintended, possibly embarrassing elements from the photo before others see it. Upload of the photo after reviewing it on a large screen is also not recommended, since most selfies suck anyway and are rarely actually worth sharing.
Having selfies be poor quality for other reasons (such as being out of focus or having motion blur due to using too slow a shutter speed in low light) is also a popular technique. Having the photo further ruined by application of bizarre and ugly filters and special effects, in order to try to conceal how poor quality the photo was in the first place, is also popular.
The term selfie is often misused. Selfies are a subgenre of self-photography. It is wrong to use "selfie" to refer to self-photographs in general.
The following methods of self-photography are NOT selfies:
1. Photographing just one's own penis does not constitute a selfie. (Even if the camera is handheld, it doesn't qualify to be called a selfie unless the photographer's face is in the photo. Putting your other "head" in the photo doesn't count.)
2. Mounting the camera on a tripod, or resting it securely on a table, and using a timer to take the photo is not a selfie. (Camera must be held by the person for it to be a selfie.)
3. Using a throwable camera that automatically snaps at panoramic photo when at the top of the throw is not a selfie.
4. Dropping your camera on the ground and being lucky enough that instead of breaking it accidentally takes a photo of you is not a selfie.
5. Mounting your camera on a wild goat and having it take many photos while it charges at you is not a selfie.