A sadhu (male), sādhvī (female) is a religious ascetic, mendicant (monk) or any holy person in Hinduism and Jainism who has renounced the worldly life. They are sometimes alternatively referred to as sannyasi or vairagi.
It literally means one who practises a ″sadhana″ or keenly follows a path of spiritual discipline. Although the vast majority of sādhus are yogīs, not all yogīs are sādhus. The sādhu is solely dedicated to achieving mokṣa (liberation), the fourth and final aśrama (stage of life), through meditation and contemplation of Brahman. Sādhus often wear simple clothing, such saffron-coloured clothing in Hinduism, white or nothing in Jainism, symbolising their sannyāsa (renunciation of worldly possessions). A female mendicant in Hinduism and Jainism is often called a sadhvi.
(Text source: Wikipedia)