Symbols:D

deci-

 * $\mathrm d$

The metric scaling prefix denoting $10^{-1}$.

deka-

 * $\mathrm {da}$

The metric scaling prefix denoting $10 \times$.

Also known as
In some older sources it is possible to see this prefix as deca-, but this is easily confused with Symbols:D and deka- is now practically universal.

Hexadecimal

 * $\mathrm D$ or $\mathrm d$

The hexadecimal digit $13$.

Roman Numerals

 * $\mathrm D$

The Roman numeral for $500$.

Integral Domains

 * $D$

Used as a variable denoting the general integral domain.

Integral Domain as an Algebraic Structure

 * $\left({D, +, \circ}\right)$

The full specification for an integral domain, where $+$ and $\circ$ are respectively the ring addition and ring product operations.

Ordered Integral Domain

 * $\left({D, +, \circ, \le}\right)$

This specifies an ordered integral domain which is totally ordered by the ordering $\le$.

Non-Zero Elements of an Integral Domain

 * $D^*$

Let $\left({D, +, \circ}\right)$ be an integral domain whose zero is $0_D$.

Then $D^*$ denotes the set $D \setminus \left\{{0_D}\right\}$.

Non-Negative Elements of an Ordered Integral Domain

 * $D_+$

Let $\left({D, +, \circ, \le}\right)$ be an ordered integral domain whose zero is $0_D$.

Then $D_+$ denotes the set $\left\{{x \in D: 0_D \le x}\right\}$, that is, the set of all positive (i.e. non-negative) elements of $D$.

Positive Elements of an Ordered Integral Domain

 * $D_+^*$

Let $\left({D, +, \circ, \le}\right)$ be an ordered integral domain whose zero is $0_D$.

Then $D_+^*$ denotes the set $\left\{{x \in D: 0_D < x}\right\}$, that is, the set of all strictly positive elements of $D$.

Some sources denote this as $D^+$, but this style of notation makes it difficult to distinguish between this and $D_+$.