Final answer:
Thrombocytopenia is the condition characterized by an abnormal decrease in the number of platelets, leading to a risk of excessive bleeding due to the inability of blood to form clots.
Step-by-step explanation:
An abnormal decrease in the number of platelets is known as thrombocytopenia. In this condition, there is an insufficient number of platelets, which typically results in the inability of blood to form clots and might lead to excessive bleeding, even from minor wounds. This is in contrast with thrombocytosis, where there is an excessive production of platelets, increasing the risk for unwanted blood clot formation known as thrombosis.