This can be affecting the appearance of the smile, and also affecting the ability to chew or eat. Nearby teeth may be beginning to fall or collapse into the space created by the missing tooth. The jaw bone where the tooth is missing may be starting to shrink away.
There is a tooth missing from the jaw.
The missing tooth may be a result of a tooth having been removed or extracted in the past; or the tooth never formed in the first place.
When a tooth is missing, the support that the missing tooth used to provide to all the other teeth is gone, which can cause all the surrounding teeth to start collapsing into the area.
Even the tooth opposing the missing tooth can come out further than usual, as the body continues to push teeth out of the jaw until they are in contact with something.
The nearby and opposing teeth will continue to collapse into the space. This can throw out the bite, and affect the cosmetic appearance of the teeth.
The lack of tooth will also result in the underlying jaw bone from dissolving away, since the jaw bone is only present to support the tooth.
Severe jaw bone resorption can result in less support for the lips and cheeks as well, creating a gaunt facial appearance.