Model.
Kepler and Newton laws are models of the real world. These are very good models due to their predictive power. Mathematical biologists derive equations (create models) of population growth and spread of epidemics. Mathematical economist create models that describe some aspects of and help study real world economy. Modeling being so important, there is of course the Model Theory. It's a general theory of interpretations of axiomatic systems. A hyperbolic (non-euclidean) geometry can be modeled in a circle of the regular euclidean geometry. This implies independence of the Axiom of Parallels (the Fifth Postulate) from the rest of the euclidean axioms.

Mean
In statistics, the measure of central tendency calculated by adding all the values and dividing the sum by the number of values. (Often referred to as the average.)

Median

Statistical
- the measure of central tendency that is in the middle when the values are arranged in order of size. If there is an even number of data items, the median is the mean of the middle two.
Of a triangle

- a line segment from any vertex of a triangle to the midpoint of the opposite side.
Mental calculation

Refers to finding an exact answer mentally (without the use of pencil and paper or calculator.)

Neighborhood.
(Set-theoretical) topology starts with a definition of open sets which are often and interchangeably called neighborhoods. By definition, the empty set is open and so are finite intersections and arbitrary unions of open sets. A set is a topological space if some of its subsets are declared to be open subject to these conditions. An open set is a neighborhood of all its points. A function f:A->B from one topological space into another is continuous at a point aA if for every neighborhood V of f(a) there exists a neighborhood U of a such that f(U)V. This is equivalent to saying that inverse images of sets open in B are open in A. Complements of open sets are closed by definition.

Numeral.
Numeral is a written symbol that denotes a number. People did not always use numerals even after learning to write. A symbol for 0 came into use only in the last millennium.

Open interval.
Open interval is a piece of a straight line that does not contain the endpoints. On the number line, open intervals (a,b) are defined as {x: a < x < b}. Open intervals are open sets.

Odd vs. Even

Odd and even are classifications of the integers. A number is even if it has remainder 0 when divided by 2; it is odd if it has remainder 1 when it is divided by 2.Even numbers have 0, 2, 4, 6, and 8 in the units or ones place. Odd numbers have 1, 3, 5, 7, and 9 in ones place.