(Sanskrit) “truth, fundamental principle.” A reference to the essential nature of a given thing.
"Prana is the Vital Christ or Great Breath. That Vital Christ is modified into Akasha, within which the Son, the First Begotten, the Purusha of every human being, is hidden. Akasha is modified into Ether, and the Ether is transformed into Tattvas. The Tattvas are the origin of fire, air, water, and earth. Therefore, everything that exists, everything that has been, and everything that shall be comes from the Great Breath, the Cosmic Christ, the Army of the Voice..." —Samael Aun Weor, Kundalini Yoga
"When you know the real Tattva (Brahman), the waking consciousness also will become quite false like a dream." —Swami Sivananda
"The names and forms (Nama-rupa), the appearances that you see outside are all effects of Maya. Maya is Avyakta (hidden, unmanifested); Avyakriti (undifferentiated). It is the indescribable power of the Lord (Anirvachaniya), being the equilibrium of Sattva (purity), Rajas (passion) and Tamas (inertia). When this equilibrium is disturbed by the will of the Lord to give fruits to the Karmas of Jivas, this world is projected at the beginning of the Maha Kalpa. Brahman thought: “There indeed are the worlds; I shall create the protectors of the worlds.” He gathered the Purusha (Hiranyagarbha) from out of the water only and fashioned him. He heated him by the heat of meditation (Aikshanta). When he was thus heated, his heart burst out, from the heat, the mind came, from the mind the moon, the presiding deity of the mind. Heart is the seat of the mind. So the mind came out when the heart burst out. In Samadhi the mind goes to its original seat, the heart. In sleep also it rests in the heart with a veil of ignorance between the mind and the Brahman” (Aitareya Upanishad 1-3-4). From the Avyaktam or the unmanifested (Maya) the Mahat Tattva comes out first, just as the sprout shoots out from the seed in the ground. From Mahat proceeds Ahankara. Then mind, senses, prana and Tanmatras, the five gross elements come into being. Then the whole external universe is created out of the five gross elements." —Swami Sivananda, Self-knowledge
In general the word tattvas refers to the five elemental forces of nature: akash (which is the elemental force of the ether), tejas (fire), vayu (air), apas (water) and prithvi (earth).
There are many other tattvas mentioned in Hinduism, but two particular higher tattvas are important: adi and samadhi.
"Those who have knowledge of the flow of the five Tattvas in the nostrils can very rapidly advance in meditation. There is an intimate connection between the mind and the five Tattvas. When Agni-Tattva flows through the nostrils, mind is much agitated and meditation is interrupted. During the flow of the Akasa-Tattva, meditation is very favourable. A knowledge of "Svara-Sadhana" or "Svarodaya" as it is popularly termed is an indispensable necessity for those who take up to meditation." —Swami Sivananda
"The physical body is composed of the five great elements or the Mahabhutas, viz., earth, water, fire, air, and ether. The Devas or gods are endowed with a divine or luminous body. The fire Tattva is predominant in them. In man, the earth Tattva is preponderating. In the case of aquatic animals, the element of water predominates. In the case of birds, the element of air predominates. Hardness of the body is due to the portion of earth; the fluidity is due to the portion of water; the warmth that you feel in the body is due to fire; moving to and fro and such other activities are due to air; space is due to Akasa or ether. Jivatma or the individual soul is different from the five elements. After death, these elements are dissolved. They reach their primordial sources in the inexhaustible storehouse of nature. The element of earth goes and joins its store of Prithvi Tattva. The other elements also go back to their sources. The respective functions of the organs are blended with the presiding gods. Sight goes to the sun from where it had its power of vision, speech goes to fire, life-breath to air, the ear into quarters, the body into the earth, hairs into annual herbs, hairs of the head into trees, and blood and semen into waters." —Swami Sivananda