A dimple is any sort of indentation, but most people use the term to refer specifically to indentations in the skin, particularly on the cheeks. Dimpled cheeks are considered attractive in many cultures, and they are a common genetic trait. Most are actually caused by a birth defect, which just goes to show that not all “malformations” are actually bad.
The most common cause of dimples is a shortened muscle, which explains why they are not always apparent when a person is at rest, since this is when the muscles are typically in their shortened state. In the face, shorter face muscles pull at the skin, especially when someone smiles, creating a classic dimple. Over time, the muscles of the face can slowly stretch out, which is why some people have them when they are young, but lose them as they age. Dimples can also form on other body parts, like the buttocks. Those in other regions are usually not as highly prized as those on the face, perhaps because the face is readily visible, while other body parts are not.
The genetics of dimples is actually rather interesting. They are a dominant trait, which means that it only takes one gene to inherit this defect. If neither parent has dimples, their children shouldn't have them either, unless they experience a spontaneous mutation. If one parent has them, the child has a 25-50% chance of inheriting the gene, since it means that parent inherited the gene from one or both parents. If both parents have dimples, the child has a 50-100% chance of inheriting the gene, depending on how the parents inherited theirs.
In most cases, facial dimples appear on the cheeks, although they can also pop up on the chin, and they are typically not visible until someone smiles because of how the muscles are used. Some people only have a dimple on one side, however, and some people find this physical trait to be rather endearing. The look of dimples can also vary; as an inherited trait, unusual ones can be passed on through multiple generations of a family.
Because dimples can fade with time as someone's facial muscles stretch out, many people associate them with youth. Children are referred to as having dimpled cheeks, for example, and someone who has them may be thought of as youthful or baby faced.