An acid (from the Latin acidus/acēre meaning sour) is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger. Chemicals or substances having the property of an acid are said to be acidic.

Common examples of acids include acetic acid (in vinegar), sulfuric acid (used in car batteries), and tartaric acid (used in baking). As these three examples show, acids can be solutions, liquids, or solids. Gases such as hydrogen chloride can be acids as well. Strong acids and some concentrated weak acids are corrosive, but there are exceptions such as carboranes and boric acid.

There are three common definitions for acids: the Arrhenius definition, the Brønsted-Lowry definition, and the Lewis definition. The Arrhenius definition states that acids are substances which increase the concentration of hydronium ions (H₃O⁺) in solution. The Brønsted-Lowry definition is an expansion: an acid is a substance which can act as a proton donor. Most acids encountered in everyday life are aqueous solutions, or can be dissolved in water, and these two definitions are most relevant. The reason why pHs of acids are less than 7 is that the concentration of hydronium ions is greater than 10⁻⁷moles per liter. Since pH is defined as the negative logarithm of the concentration of hydronium ions, acids thus have pHs of less than 7. By the Brønsted-Lowry definition, any compound which can easily be deprotonated can be considered an acid. Examples include alcohols and amines which contain O-H or N-H fragments.

The Moon is the only natural satellite of the Earth, and the fifth largest satellite in the Solar System. It is the largest natural satellite of a planet in the Solar System relative to the size of its primary, having a quarter the diameter of Earth and ¹⁄₈₁ its mass. The Moon is the second densest satellite after Io, a satellite of Jupiter. It is in synchronous rotation with Earth, always showing the same face; the near side is marked with dark volcanic maria among the bright ancient crustal highlands and prominent impact craters. It is the brightest object in the sky after the Sun, although its surface is actually very dark, with a similar reflectance to coal. Its prominence in the sky and its regular cycle of phases have, since ancient times, made the Moon an important cultural influence on language, calendars, art and mythology. The Moon's gravitational influence produces the ocean tides and the minute lengthening of the day. The Moon's current orbital distance, about thirty times the diameter of the Earth, causes it to appear almost the same size in the sky as the Sun, allowing it to cover the Sun nearly precisely in total solar eclipses.

Guru (Devanagari गुरु) is a Sanskrit term for "teacher" or "master", especially in Indian religions. The Hindu guru-shishya tradition is the oral tradition or religious doctrine transmitted from teacher to student.

In the United States, the meaning of "guru" can be used to cover anyone who acquires followers, especially by exploiting their naiveté, due to the inflationary use of the term in new religious movements.

The syllable gu means shadows
The syllable ru, he who disperses them,
Because of the power to disperse darkness
the guru is thus named.

— Advayataraka Upanishad 14—18, verse 5

The word guru, a noun, means "teacher" in Sanskrit and in other languages derived from or borrowing words from Sanskrit, such as Hindi, Tamil, Telugu,Kannada, Malayalam, Marathi, Bengali, Gujarati and Nepali.The malayalam term Acharyan or Asan are derivered from the Sanskrit word Acharya. It is transliterated in different ways such as Asaan, Ashan, Aasaan etc.

As a noun the word means the imparter of knowledge (jñāna; Also Persian: Dāna). As an adjective, it means 'heavy,' or 'weighty,' in the sense of "heavy with knowledge," heavy with spiritual wisdom, "heavy with spiritual weight," "heavy with the good qualities of scriptures and realization," or "heavy with a wealth of knowledge." The word has its roots in the Sanskrit gri (to invoke, or to praise), and may have a connection to the word gur, meaning 'to raise, lift up, or to make an effort'.