Definitive seriesA definitive postage stamp is a regular issue stamp that is part of a definitive issue or definitive series consisting of a range of denominations sufficient to cover all postal rates usefully. (An "issue" generally means a set that is put on sale all at the same time, while a "series" is spread out over several years, but the terms are not precise.)
The term came into general use after World War I, to differentiate the new countries' regular stamps from the provisional issues that had been in use in many places. Previously such stamps were simply referred to as "general" or "regular" issues, or simply not identified as such, but the issuance of commemorative stamps and "special issues" that had begun in the 1890s had left to a confused situation where countries were issuing large numbers of stamps that saw little actual use.
The range of values varies by era and country, but the focus is on coverage; the values should be sufficient to make up all possible charges using as few stamps as possible. Generally the smallest value will be the smallest unit of currency, or smallest fractional postal rate; for instance, the 1954 Liberty issue of the United States included a 1/2-cent stamp because some rates were expressed in fractions of a cent per ounce. The highest value of the series is generally quite large, typically from 50-100 times the normal letter rate.
The in-between values are usually chosen to "make change" efficiently, for instance 1, 2, 5, 10, 20, 25, 50 in a decimal currency. It is common to include all values between 1 and 10, multiples of 5 from 10 to 50, and multiples of 10 from 50 to 100. Additional "oddball" values may reflect specific common rates, and if the series lasts for a period of time, there may be a number of such unusual values.
Definitives are the workhorse stamps of a country, and as such, they tend to be small, with designs reflecting local culture and history. The definitives of poorer countries will often be very plain and cheaply printed, unlike the large and decorative commemoratives, which are almost pure profit if bought by foreign collectors and never used for postage.
Since postal administrations know that stamp collectors want to own every stamp of a definitive series, and a complete series can be quite expensive, there is always the temptation to make some extra money by issuing new definitive sets. Collectors' organizations have recommended that administrations only bring out new definitive issues no more often than every five years, and most administrations of the world follow this policy. An exception would be the death of a monarch, necessitating a new definitive series for the new ruler.