| data (1) Information gathered by observation, questioning, or measurement.
deca- Prefix meaning ten.
decimal A number written in standard notation, usually one containing a decimal point, as in 2.54.
decimal point (2) The period which separates the whole number from the fraction in decimal notation. In expressing money, it separates the dollars from the cents.
decimeter (dm) (3) In the metric system, a unit of length equivalent to 10 centimeters or of a meter.
deficient number (4) A number in which the sum of all its proper factors is less than the number.
Example: 10 is a deficient number because the sum of its proper factors is 1 + 2 + 5 = 8 and 8 is less than 10.
degree (°) (1) a) A unit of measure for angles; based on dividing a circle into 360 equal parts; b) A unit of measure for temperature.
denominator (1) The number of equal parts into which the whole (or ONE or unit) is divided. In the fraction  , b is the denominator or the number written below the line.
diagonal (1) An arrangement of objects or numbers from upper left to lower right or from lower left to upper right in an array or table.
diameter (3) A line segment that passes through the center of a circle (or sphere) and has end-points on the circle (or sphere); also, the length of such a line segment. The diameter of a circle is twice its radius.
difference (1) The amount by which one number is greater or less than another.
digit (1) In the base-10 numeration system, one of the symbols 0, 1, 2, 3, 4, 5, 6, 7, 8, 9. Digits can be used to write any number.
Example: The numeral 145 is made up of the digits 1, 4, and 5.
digital clock (1) Clock that uses numbers only to show the time in hours and minutes, with a colon used to separate them.
dimension (4) A measure in one direction, for example length or width or height.
dividend (4) In division, the total to be divided by the divisor. See division.
divisibility test (4) A test to determine whether a whole number is divisible by another whole number, without actually doing the division. For example, to tell whether a number is divisible by 3, check whether the sum of its digits is divisible by 3.
Example: 51 is divisible by 3 since 5 + 1 = 6, and 6 is divisible by 3.
divisible by (4) One whole number is divisible by another whole number if the result of the division is a whole number with a remainder of zero.
Example: 28 is divisible by 7, because 28 divided by 7 is 4 with a remainder of zero
division (2) A mathematical operation based on "sharing" or "separating into equal parts." The dividend is the total before sharing. The divisor is the number of equal parts or the number in each equal part. The quotient is the result of division. The remainder is the number left over when a set of objects is shared equally or separated into equal groups.
Example: In  = 4R1, 29 is the dividend, 7 is the divisor, 4 is the quotient and the remainder is 1.
divisor (4) If a number n is divisible by a number x, then x is a divisor of n.
Example: 21 is divisible by 3, so 3 is a divisor of 21.
doubles fact (1) The addition and multiplication facts without turn-around partners. A doubles fact names the sum or product of a 1-digit number added to or multiplied by itself.
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