India has had a long value based Guru-Shishya Parampara (teacher-disciple tradition) since ancient age. The nearest literal English meanings of Guru is teacher, but a host of other Sanskrit/Pali terms like Acharya, Upadhyay Shishya have been alternatively used. Though in common parlance they often used as a substitute for each other, but in essence each has distinct meaning, purpose and implication. This Parampara has remained a unique feature of all Indian religions viz. Hinduism (Sanatana Dharma), Jainism, Buddhism, and even in the much later evolved Sikhism all along the course of their history. There is enough evidence in Hindu texts to suggest that the Guru-Shishya Parampara had evolved as a tradition in the Vedic culture and passed on to and nurtured by all other Indian religions.

Here is an attempt to briefly explain some of the more common terms still in use:

Teacher: In common parlance, a teacher is one who helps pupils to acquire worldly knowledge, skills and even virtues. Ordinarily, the term is used for the paid professionals but loosely applicable to anyone who formally or informally disseminate it any class of people:

Guru: In Hinduism, the Guru (Sanskrit term) is a central figure with multiple roles such as a teacher of material knowledge and skills, a counsellor of values and experiential knowledge, an exemplar and inspiration that guides the seeker through spiritual development, knowledge of the supreme truth and realization of one's soul. Guru and his role finds a mention in many Principal Upanishads such as Katha Upanisad, Chandogya Upanishad, Taittiriya Upanishad, etc., but the Bhagavad Gita summarizes it briefly as under:

तद्विद्धि प्रणिपातेन परिप्रश्नेन सेवया I
उपदेक्ष्यन्ति ते ज्ञानं ज्ञानिनस्तत्त्वदर्शिन:II

(Learn the Truth by approaching a spiritual master. Inquire from him with reverence and render service unto him. Such an enlightened Saint can impart knowledge unto you because he has seen the Truth.) (BG: Chapter 4, Verse 34)

Acharya: It represents a title and expertise of a person who is highly learned having achieved a certain level of knowledge and proficiency, mostly used in the religious context in the ancient India but now loosely also used for an expert teacher/scholar in any discipline (A Sanskrit term). In Hinduism, it has been used as a formal title of many teachers or gurus having attained high degree of knowledge in Vedas and allied literature. Ex. Adi Shankaracharya, Ramanujacharya

Upadhyay: Another Sanskrit term used for the person who studied and has good knowledge of the Upanishads. In Gurukul system, it was also used for such teachers and is a surname of some Brahmins too in present days.

The usual characteristic of the ancient tradition was that the disciple would live with the Guru in a spiritual, intellectual and emotional bonding and eventually master the knowledge embodied by him, and even acquire title as applicable to his knowledge and learning level.