Final answer:
Thrombocytopenia is an abnormal decrease in the number of platelets in the blood, causing an inability to form clots and leading to excessive bleeding. The opposite condition is thrombocytosis, causing an increased risk of thrombosis.
Step-by-step explanation:
An abnormal decrease in the number of platelets circulating in the blood is known as thrombocytopenia.
This condition typically results in the blood's inability to form clots, which can lead to excessive bleeding even from minor injuries.
Disorders such as aplastic anemia may contribute to reduced platelet production, among other causes like radiation, certain medications, chemotherapy, or infections.
The opposite condition, when there is an excessive number of platelets, is called thrombocytosis, which increases the risk of forming unwanted blood clots, or thrombosis.