Decision variables

declarations
  x, b, d: mpvar
  ifmake: array(1..10, 1..20) of mpvar
  y, z: array(1..10) of mpvar
end-declarations

mpvar means mathematical programming variable or decision variable, sometimes also just called variable. Decision variables are unknowns: they have no value until the model is run, and the optimizer finds values for the decision variables.
Variables can take values between 0 and infinity by default, other bounds may be specified:

x <= 10
y(1) = 25.5
y(2) is_free
z(2,3) >= -50
z(2,3) <= 50

Integer programming types are defined as unary constraints on previously declared decision variables

b is_binary                      ! Single binary variable
forall(p in PRODS,l in LINES)
  ifmake(p,l) is_binary          ! An array of binaries
d is_integer                     ! An integer variable
d <= 25                          ! Upper bound on the variable
x is_partint 10                  ! Partial integer (integers up to 10, continuous beyond)
y(3) is_semcont 5                ! Semi-continuous (0 or greater or equal 5)