The terminology used in Fits & Tolerances is shown below.
Basic size : It is also called as "Nominal size". It is the exact theoretical size arrived at designing. This is the size which is obtained by calculation for strength. It is the size from which limits or deviations are assigned.
Actual size : Actual size is the dimension as measured on a manufactured part.
Deviation : It is the algebraic difference between the basic size and the hole or shaft size.
Upper deviation : It is the difference between the basic size and permitted maximum size of the component. It is a positive quantity when the maximum limit of size is greater than basic size and vice-versa.
Lower deviation : It is the difference between the basic size and the minimum permitted size of the part.
Mean deviation : It is the arithmetic mean deviation between the upper deviation and lower deviation.
Zero line : It is the line of zero deviation and represents the basic size. When the zero line is drawn horizontally, positive deviations are shown on above and negative deviations are shown on below the line.
Fundamental deviation : It is the deviation closest to the basic size.
Limits of size : These are the maximum and minimum permissible sizes of the part.
Tolerance : It is the difference between permitted maximum and minimum sizes of the part.
Tolerance zone : It is the zone bounded by the two limits of size of the parts and defined by its magnitude i.e., tolerance and by its position in relation to the zero line.
Allowance : It is an international difference between maximum and minimum limits of mating parts. It is the minimum clearance or maximum interference between mating parts.
Unilateral limits : In this type of method of presenting the limits, both the limits of size are on the same side of zero line. Ex: 
or 
e
Bilateral limits : In this type of representation, one of the limits of size one side of the zero line and the other limit of size is on the other side of the zero line. Ex: 
..