constant
A constant is a quantity that holds still. While a variable is free to take different values, a constant keeps the same value throughout the expression or problem you are working in. The plain number 7 is a constant; so is π, whose value (about 3.14159) never changes no matter what circle you draw.
Constants come in two flavours. Some are literal numbers written out, like the 5 in “5 + x.” Others are letters used to name a fixed but unspecified value — the a, b, c in “ax^2 + bx + c.” These look like variables, but within a single problem they are pinned down: they label numbers that stay fixed while x is the thing that moves.
Telling constant from variable is about role, not appearance. The same symbol can be a constant in one discussion and a variable in another. The reliable test is to ask: in this problem, is this quantity allowed to change, or is it held fixed? If it is held fixed, it is acting as a constant.
In 3x + 4, the 3 and the 4 are constants; x is the variable. No matter what x becomes, the 3 and 4 stay 3 and 4.
The fixed numbers are the constants; the moving letter is the variable.
Famous named constants — π ≈ 3.14159 and e ≈ 2.71828 — are irrational numbers with fixed, never-changing values, written with letters only for convenience.