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.

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The soul – in many traditional spiritual, philosophical, and psychological traditions – is the incorporeal and immortal essence of a person, living thing, or object. Some religions teach that all biological organisms have souls, and others further still that even non-biological entities (such as rivers and mountains) possess souls. This latter belief is called animism.Anima mundi and the Dharmic Ātman are concepts of a "world soul." Some religious thinkers, such as Thomas Aquinas, attribute souls to all organisms but teach that only human souls are immortal.

Soul can function as a synonym for spirit, mind or self; scientific works, in particular, often consider 'soul' as a synonym for 'mind'^{[citation needed]}.

The Modern English word soul derived from Old English sáwol, sáwel, first attested to in the 8th century poem Beowulf v. 2820 and in the Vespasian Psalter 77.50, and is cognate with other Germanic and Baltic terms for the same idea, including Gothic saiwala, Old High German sêula, sêla, Old Saxon sêola, Old Low Franconian sêla, sîla, Old Norse sála as well as Lithuanian siela. Further etymology of the Germanic word is uncertain. A more recent suggestion connects it with a root for "binding", Germanic *sailian (OE sēlian, OHG seilen), related to the notion of being "bound" in death, and the practice of ritually binding or restraining the corpse of the deceased in the grave to prevent his or her return as a ghost.