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.

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.

Addition : Addition is the mathematical operation that describes increasing a number by an amount equal to a second number. The mathematical symbol for addition is the plus sign $+.$ The term addition is also used for a generalization of this basic operation on numbers to functions, vectors and matrices.

Comparing coefficients : Comparing coefficients means to check if all the coefficients of two polynomials coincide. For example to check if a polynomial equals zero you can equivalently check if all of its coefficients are zero.

Division : Division is the mathematical operation that divides objects equally into groups. More generally $x\div y$ is defined as the number that if multiplied by $y$ equals $x.$

Equation : An equation is a mathematical statement in which two expressions are written with an equal sign in between. A solution of an equation is a set of variables that makes the statement a true statement.

Solution : A solution to an equation is a choice of variables that when substituted into the equation results in a true statement.