Not having sufficient insurance to cover loss or damage(adjective)
Not having proper health insurance(adjective)
One who has insufficient insurance.(noun)
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Use "underinsured" in a sentence
"On top of that, 25 million Americans are also deemed "underinsured" - their coverage is inadequate for their needs."
"What is clear is that in a new marketplace which would be available to those Americans without insurance coverage right now at all or those Americans who have -- or the so-called underinsured, who have limited coverage, they need affordable coverage, so a new marketplace that will be primarily private plans offering a series of benefits, and, as a competitive ingredient, you would have a public option."
"The underinsured is a group that we're focusing on much more."