Sadhu (Sanskrit) [from the verbal root sadh to finish, perfect, complete, overcome, conquer] Feminine sadhi. A good and virtuous man; more particularly a holy man; especially with the Jains, a jina or deified saint. As an adjective, completed, perfected, hence accomplished; successful, effective (in regard to hymns), excellent, good, fit, proper. As an interjection, excellent! Well done! Good!
In Hinduism, a sādhu (Sanskrit , "good; good man, holy man") is a religious ascetic or holy person. 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 saffron-coloured clothing, symbolising their sannyāsa (renunciation). This way of life is open to women; the female form of the word is sādhvī. In 2014, an all-female akhada (group of sadhus) was formed; it is believed to be the first such group in India.
Dictionary source: WordNet 2.0
English to English translation of sadhu
This is essentially the same as a sannyasin . This is a person who renounces life and everything that goes with it (religion, caste, family, etc.) and essentially becomes a wandering hermit seeking moksha .