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This tag should be applied to questions concerning acid and base reactions. An acid is capable of donating a hydron/ proton (Brønsted acid) or capable of forming a covalent bond with an electron pair (Lewis acid). A base on the other hand is a chemical species/ molecular entity having an available pair of electrons capable of forming a covalent bond with a hydron/ proton (Brønsted base) or with the vacant orbital of some other species (Lewis base).

This tag should be applied to questions concerning acid and base reactions.

According to the IUPAC goldbook, an acid is a molecular entity or chemical species capable of donating a hydron (proton) (Brønsted acid) or capable of forming a covalent bond with an electron pair (Lewis acid).

Brønsted acid (source)
A molecular entity capable of donating a hydron (proton) to a base, (i.e. a 'hydron donor') or the corresponding chemical species. For example: $\ce{H2O, H3O+, CH3CO2H, H2SO4, HSO4^{−}, HCl, CH3OH, NH3}$ .

Lewis acid (source)
A molecular entity (and the corresponding chemical species) that is an electron-pair acceptor and therefore able to react with a Lewis base to form a Lewis adduct, by sharing the electron pair furnished by the Lewis base. For example:
$\ce{\underset{Lewis~acid}{(H3C)3B} + \underset{Lewis~base}{:NH3} -> \underset{Lewis~adduct}{(H3C)3\overset{\small{~~\ominus}}{B}-\overset{\small{\oplus}}{N}H3}}$

In conjunction to this, the definition of a base is a chemical species or molecular entity having an available pair of electrons capable of forming a covalent bond with a hydron (proton) (Brønsted base) or with the vacant orbital of some other species (Lewis base).

Brønsted base (source)
A molecular entity capable of accepting a hydron (proton) from an acid (i.e. a 'hydron acceptor') or the corresponding chemical species. For example: $\ce{{}^{-}OH, H2O, CH3CO2^{−}, HSO4^{−}, SO4^{2−}, Cl^{−}}$.

Lewis base (source)
A molecular entity (and the corresponding chemical species) able to provide a pair of electrons and thus capable of coordination to a Lewis acid, thereby producing a Lewis adduct.

The two kinds of molecules or chemical species are closely related as they form so called conjugated acid-base pairs in the Brønsted sense, or as mentioned previously, as a Lewis adduct in the alternative Description.

conjugate acid–base pair
The Brønsted acid $\ce{BH+}$ formed on protonation of a base $\ce{B}$ is called the conjugate acid of $\ce{B}$, and $\ce{B}$ is the conjugate base of $\ce{BH+}$. (The conjugate acid always carries one unit of positive charge more than the base, but the absolute charges of the species are immaterial to the definition.) For example: the Brønsted acid $\ce{HCl}$ and its conjugate base $\ce{Cl^{−}}$ constitute a conjugate acid–base pair.

The reactivity of acids and bases is dependent on the $\mathrm{p}\ce{H}$ of a solution and as a consequence, every reaction of acids and bases will change this property of a solution.

In aqueous solution at $T = 25~^\circ\mathrm{C}$, acids usually have a $\mathrm{p}\ce{H}$ less than 7 and bases have a $\mathrm{p}\ce{H}$ greater than 7, this is because the neutrality of an aqueous solution is determined or governed by the autoprotolysis of water. $$\ce{2H2O <=> H3+O + {}^{-}OH}$$ Therefore neutrality of a solution is achieved when the activities of hydronium and hydroxide ions are equal, $$a(\ce{H3+O})=a(\ce{{}^{-}OH}),$$ or in simpler terms, when the concentrations of hydronium and hydroxide ions are equal, i.e. $$[\ce{H3+O}]=[\ce{{}^{-}OH}].$$

Due to this nature these questions are usually related to and/or .

A fairly good answer, that covers acids and bases in general can be found in this question.