In philately, the denomination is the "inscribed value of a stamp". For instance, if you visit the post office to buy a stamp to pay $1's worth of postage you will receive a stamp that has the value $1 printed on it in words or numbers.
The denomination is not the same as the value of a stamp on the philatelic market, which is usually different, and the denominations of a country's stamps and money do not necessarily match. For instance, there might be a 47c stamp to pay a particular postal rate but there is unlikely to be a 47c coin.
Where no denomination is shown, it may be because the stamp is deliberately non-denominated to pay the cost of a particular service, or because the stamp is not a postage stamp. It might be a cinderella stamp of some kind such as a poster stamp or charity label.
Sometimes a stamp may have its denomination changed by the post office due to local circumstances. For instance, stocks of one value may be overprinted to show a different value due to stock shortages. In cases of hyper-inflation stamps have had their denomination changed by overprinting as existing denominations became worthless. In other cases, changes to the local currency have led to changes in denomination. For instance, when the Ryukyu Islands (at the time a United States protectorate) changed its currency from Yen to Dollars, a number of airmail stamps originally printed with Yen values were overprinted and re-denominated to cents in 1959–1960.