Hole in tooth - is this a cavity?

Cavity: Most likely the hole in your tooth is a cavity. If you can see it it is most likely a large cavity. I recommend you see your dentist asap. If a cavity progress you may get a toothache and infection.

Literally: A cavity of any kind is a hole. A cavity in a tooth (carie in techspeak) is a hole, a break in the enamel (tough outer coating), can get deeper into pulp, causing pain, infection, get into root of tooth and surrounding tissue-bone, soft tissue. See a dentist, get it fixed, sooner rather than later. Prevent them with good oral hygiene, floss, brush, no sugary sweets, don't do meth; check meds w/md.

Probably: But there are other causes such as developmental defects or fractures. The only way to be sure is to be physically examined by a General Dentist. If it is a cavity it is easier and less expensive to treat it when it's small.

Decay: Yes. It's another name for decay/cavity/caries.

Usually.: If you have noticed a change in a particular tooth, ie you have a "hole" that was not present previously, it is most likely caused by decay of the tooth. See a dentist for diagnosis and treatment as soon as possible to prevent pain and possible loss of the tooth.