The clinical definition is acute leukemia can cause death within a few
months without treatment and chronic leukemia takes longer or is more
chronic.

The laboratory diagnosis definition of acute leukemia is dependent on
the number of leukemia cells (blast cells)  found in the bone marrow.
More than 30 % constitutes acute leukemia where they usually only
comprise about 5% normally.
The blast cell is the hallmark of acute leukemia whereas other more
mature cells are the hallmark of chronic leukemia. The type of leukemia
is named after the malignant cell responsible for this transformation.