A rare book is a book that is considered scarce or valuable due to its age, historical significance, limited edition, unique features, or other distinguishing factors. These books are often sought after by collectors, scholars, and enthusiasts.

The rarity of a book can be determined by several factors, including:

1. Age: Books that are several centuries old are generally considered rare due to the natural attrition of older books over time.

2. Scarcity: Books that were printed in limited quantities, such as first editions or books from small or private presses, are often rare and more valuable.

3. Historical significance: Books that played a pivotal role in history, contain important ideas, or document significant events are highly sought after. For example, a first edition of a classic literary work or a book associated with a famous historical figure would be considered rare.

4. Unique features: Books that possess special attributes can be considered rare. This may include rare bindings, illustrations, or annotations by notable individuals.

5. Condition: Books that are in excellent condition, particularly if they retain their original dust jackets or have been well-preserved, are more valuable than those in poor condition.