Button batteries refer to small, round, and flat batteries that often resemble buttons. They are used to power small electronic devices like wristwatches, calculators, and key fobs. Full definition
Because it has no sophisticated electronics inside, it's also said to last as long as 60 days on a single button battery. (theverge.com)
Thousands of children are admitted to the ER each year because of swallowed button batteries. (momtricks.com)
Many contain batteries which can be dangerous to small children and some contain lithium button batteries which can be fatal if swallowed by unsuspecting children. (health-and-parenting.com)