Definition:
Quite simply, artistic license is doing whatever you wish in a painting, without regard to artistic conventions, art critics, theories, or history, or what reality looks like. It's applying your own vision and views in your work, without worrying about whether it's realistic or not, nor about what other people may think or say.
You can apply it in varying degrees, from only a little to oodles. For example, leaving out a few trees in a landscape painting if you don't want them there or moving a tree to another position. Using bright greens or purples in a portrait rather than realistic skin tones. When you eliminate detail and create abstractions; when you invent, imagine, and elaborate. It's painting however and whatever you desire, simply because you want it that way.
It can be something deliberate and consistent, something that defines a style. For instance the types of color choices Fauvists made, which enhance dramatic effect and visual impact, or a particular form of mark making you use a lot. It can be the level of detail included. Repositioning things in a landscape or combining scenes for a stronger composition. It can even be accidental or unintentional, something you'd not noticed and thus omitted, something you've perceived differently because of light conditions or your eyesight.
Artistic license is the "creative juice" that comes from the artist's mind, the interpretation and selection of what to convey and how. It isn't something you buy, it's something you grant yourself. Freedom of expression, to experiment and see what happens, to do it the way you want it to be, without worrying about whether it's right or wrong, whether it's tasteful or acceptable, whether it will sell or not. It's a license to create.
