【Hole】 ,  【hollow】 ,  【cavity】 ,  【pocket】 ,  【void】 ,  【vacuum】  are comparable when they mean an open or unfilled space in a thing.

【Hole】 may apply to an opening in a solid body that is or that suggests a depression or an excavation or to one that passes through the material from surface to surface.

【Hollow】 , which specifically implies opposition to solid, basically suggests an unfilled space within a solid object, usually one that has a surface opening. The term, however, is often applied to a depression in a surface or to a deep and narrow valley (as a gully or ravine).

【Cavity】  is a somewhat more learned word than  【hollow】 with much the same implications as the latter in its basic sense. The words are often used interchangeably, but  【cavity】 is preferred in technical use.

【Pocket】  is often employed in place of  【cavity】 for an abnormal or irregular space (as a bubble-like one in a substance or a sack-like one in a body). It is particularly referred to one that is a source of danger, especially in possessing the tendency to hold or to collect a foreign substance (as dirt, air, or pus).

【Void】  applies to an apparently empty space, especially one of marked extent or of conspicuous duration, whether in a thing that is normally continuous <the air-filled voids of the soil —A. M. Bateman >  or between things that are normally separate.

【Vacuum】 basically and especially in technical use applies to space entirely devoid of matter; more often, however, it is applied to the space within an enclosed vessel in which by mechanical means the air has been practically, though seldom completely, exhausted. In its extended use the term applies to a condition or situation which resembles a true 【vacuum】 in its emptiness of all that normally should fill it or exert influence on anyone or anything that remains in it.