| Term | Definition | | Variance | A measure of how different the numbers in a set are from one another. For example, the two sets of numbers {6, 2, 3, 1} and {2, 4, 3, 3} have the same average (or mean) of 3, but the first set has a greater variance. |
| Viral Vector Vaccine | A vaccine that uses a modified, harmless fraction of a different virus (a vector virus) associated with an antigen that can induce production of important instructions that are delivered to the body's cells. |
| Virulence | The relative capacity of a pathogen to overcome body defenses and elicit symptoms. |
| Virus | A tiny organism that multiplies within cells and causes diseases such as chickenpox, measles, mumps, rubella, and hepatitis. Viruses are not affected by antibiotics, the drugs used to kill bacteria. |
| Water Vapor | The most abundant greenhouse gas, it is the water present in the atmosphere in gaseous form. Water vapor is an important part of the natural greenhouse effect. While humans are not significantly increasing its concentration through direct emissions, it contributes to the enhanced greenhouse effect because the warming influence of greenhouse gases leads to a positive water vapor feedback. |
| Weather | Weather is the day-to-day, short-term state of the atmosphere at a specific time and place, defined by variables like temperature, humidity, precipitation, wind, and atmospheric pressure. |
