Is half life the term for the activity of a radioisotope to decrease in half?

No, half-life refers to the time it takes for half of the atoms in a radioactive substance to decay, not the activity itself. Activity is a measure of the number of decays per unit time, which decreases as the amount of the radioactive material diminishes. Thus, while half-life provides a timeframe for decay, it does not directly describe the activity level.