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$\mathrm a$

The Système Internationale d'Unités symbol for the metric scaling prefix atto, denoting $10^{\, -18 }$, is $\mathrm { a }$.

Its $\LaTeX$ code is \mathrm {a} .


$\mathrm a$

The are is a measure of area:

One are is equal to a square whose side measures $10$ metres.

\(\displaystyle \) \(\) \(\displaystyle 1\) are
\(\displaystyle \) \(=\) \(\displaystyle 100\) square metres
\(\displaystyle \) \(\approx\) \(\displaystyle 119 \cdot 60\) square yards


$\mathrm A$ or $\mathrm a$

The hexadecimal digit $10$.

Its $\LaTeX$ code is \mathrm A  or \mathrm a.

Celestial Altitude


Let $X$ be a point on the celestial sphere.

The (celestial) altitude of $X$ is defined as the angle subtended by the the arc of the vertical circle through $X$ between the celestial horizon and $X$ itself.

It is often denoted by $a$.

Azimuth (Astronomy)


Let $X$ be a point on the celestial sphere.

The spherical angle between the principal vertical circle and the vertical circle on which $X$ lies is the azimuth of $X$.

The azimuth is usually measured in degrees, $0 \degrees$ to $180 \degrees$ either west or east, depending on whether $X$ lies on the eastern or western hemisphere of the celestial sphere.

It is often denoted by $A$.

Alternating Group


The alternating group on $n$ letters.



Prefix indicating anti.

An abbreviation for arc, anti- or argument.



Prefix indicating not.


Abbreviation for the antilogarithm of a general logarithm.
If no base is given, the common logarithm (that is, base $10$) is assumed.