Any tooth (but especially a wisdom tooth) that cannot break through the gum into normal position because it is growing against or blocked by bone, soft tissue, or another tooth.
Teeth start to pass through the gums (emerge) during infancy, and again when the primary (baby) teeth are replaced by the permanent teeth.
If a tooth fails to emerge, or emerges only partially, it is considered to be impacted. The most common teeth to become impacted are the wisdom teeth (the third set of molars). They are the last teeth to emerge, usually between the ages of 17 and 21.