Notice that not all prices rise during an inflation. Some prices may be falling, but those prices which are rising tend to pull up the general level of prices. Deflation is a decrease in the general level of prices, as measured by some price index. Notice that not all prices fall during an deflation. Some prices may actually be rising, but those prices which are falling tend to pull down the general level of prices.
Disinflation is a decrease in the rate of inflation.
Disdeflation is a decrease in the rate of deflation.