If you draw a picture of yourself, it's a self-portrait. Vincent van Gogh was well-known for his many self-portraits, painting more than 40 of them over the course of a few years.
Whenever an artist creates a portrait of herself, whatever medium she uses, the result is a self-portrait. A six year-old can draw a self-portrait in crayon, and a famous sculptor can sculpt one out of clay. In the mid-15th century, Early Renaissance painters were the first to focus on creating deliberate self-portraits; this was probably due partly to the fact that mirrors had just recently become affordable and fairly easy to find.