What is a condition characterized by excessive carbon dioxide in the blood?

Hypercapnia is the correct answer as it specifically refers to elevated levels of carbon dioxide (CO2) in the bloodstream. This condition occurs when there's an imbalance in the production and elimination of CO2, often due to respiratory issues or impaired gas exchange within the lungs. An increase in CO2 can lead to respiratory acidosis as the carbonic acid generated can decrease blood pH, which is related to the process that ultimately affects oxygen delivery to tissues.

Hypoxemia refers to low levels of oxygen in the blood rather than increased carbon dioxide. Hypoxia indicates insufficient oxygen supply to the tissues, which could result from several conditions but does not focus on carbon dioxide levels. Acidosis is a broader term that describes a decrease in pH of the blood and may be a consequence of hypercapnia, but it is not directly indicative of elevated carbon dioxide levels itself. Therefore, hypercapnia is the most precise term for excessive carbon dioxide in the blood.