Musk is a class of aromatic substances commonly used as base notes in perfumery. They include glandular secretions from animals such as the musk deer, numerous plants emitting similar fragrances, and artificial substances with similar odors. Musk was a name originally given to a substance with a strong odor obtained from a gland of the musk deer. The substance has been used as a popular perfume fixative since ancient times and is one of the most expensive animal products in the world. The name originates from the Late Greek μόσχος 'moskhos', from Persian 'mushk', similar to Sanskrit मुष्क muṣka ("testicle"), derived from Proto-Indo-European noun múh₂s meaning "mouse". The deer gland was thought to resemble a scrotum. It is applied to various plants and animals of similar smell and has come to encompass a wide variety of aromatic substances with similar odors, despite their often differing chemical structures and molecular shapes.