Activity is the way of measuring the number of nuclear disintegrations per unit of time in a quantity of a radioactive substance. The modern unit of activity is the becquerel (Bq) . One becquerel represents one decay per second. Sample of plutonium-239 with an activity of 2,000
megabecquerals will emit an average of 2,000 million alpha particles every second. One curie= 37,000 million becquerels. Since a piece of radioactive material's number of becuerels will change over time, a sample of radioactive decay rate will always be stated with a timestamp for short-lived isotopes with an adjustment made to some specific date. For example, a ten-day figure could be given to indicate how much activity will be present ten days in the future.

References

May, John. The Greenpeace Book of the Nuclear Age: the Hidden History, the Human Cost. New York: Pantheon, 1989

http://www1.bipm.org/en/si/derived_units/2-2-2.html