Count : To count means to find out the number of objects by adding one for every object.

Number : A number $x$ is a mathematical symbol representing a quantity.

Estimation : Estimation is another word for finding a close but not necessarily exact solution to an arithmetic problem.

Number : A number $x$ is a mathematical symbol representing a quantity.

Ones digit : The first digit left of the decimal point (or the digit furthest to the right if there is no decimal point). It counts multiples of 1.

Rounding : Rounding is the operation of changing a number to a number that is close to the original number. Rounded numbers are often used to do quick rough calculations for estimation purposes. For example rounding $4549$ to the nearest thousand results in 5000, rounding the same number to the nearest hundred results in 4500. In general one rounds up if the digit after the digit we try to round to is 5 or more, otherwise one rounds down.

Significant figure : Rounding a number to k significant figures means that one leaves the first $k-1$ digits unchanged and rounds the k-th digit up if the (k+1)-th digit is at least 5 and rounds it down otherwise. For example rounding the number 1234 to 2 significant digits results in 1200.

Thousandths digit : The thousandths digit is the third digit after the decimal point. It counts multiples of $\frac{1}{1,000}.$

Centimeter : A centimeter ($cm$) is $\frac{1}{100}$ of a meter.

Factor : If a natural number is divisible by a second natural number, that second natural number is called a factor of the first natural number. For example the factors of 12 are 1,2,3,4,6 and 12.

Length : Length is the attribute of a one-dimensional shape that can be measured with a measuring tape.

Line : A line AB is a one-dimensional shape that includes the points A and B, all the points on the line segment in between A and B and all the points of the straight extension of the line segment beyond A and B. A line does not have an endpoint.

Scale : Scale refers to the size of a shape. A scale drawing changes a shape to a similar shape of a different size.