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

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

Hundredths digit : The hundredths digit digit is the second digit after the decimal point. It counts multiples of $\frac{1}{100}.$

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

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.

Tenth : Tenth refers to the number $\frac{1}{10}=0.1=10^{-1}.$

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

Absolute value : The absolute value $|x|$ of a number $x$ is defined as the value of $x$ ignoring any potential minus sign. For example $|-2|=2$ and $|2|=2.$ In other words $|x|=x$ for $x\geq 0$ and $|x|=-x$ for $x\lt 0.$