Viruses are constantly evolving and changing. Every time a virus replicates (makes copies of itself), there is the potential for there to be changes in its structure. Each of these changes is a “mutation.” A virus with one or more mutations is called a “variant” of the original virus. 

Some mutations can lead to changes in important characteristics of the virus, including characteristics that affect its ability to spread and/or its ability to cause more severe illness and death.