1
: a type of dangerous and powerful energy that is produced by radioactive substances and nuclear reactions
She was exposed to high levels of radiation.
2
medical
: the use of controlled amounts of radiation for the treatment of diseases (such as cancer)
He goes in for radiation next week.
called also radiotherapy
3
a
: energy that comes from a source in the form of waves or rays you cannot see
ultraviolet radiation
b
technical
: the process of giving off energy in the form of waves or rays you cannot see
the sun's radiation of heat
Cite this Entry
“Radiation.” Merriam-Webster.com Simple Definition, Merriam-Webster, https://www.merriam-webster.com/simple/radiation. Accessed 18 Jun. 2026.
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