1. to be the basis or foundation for something; to form the underlying cause or reason
2. to lie or be situated beneath something

Examples

• Economic factors underlie most political decisions.
• A deep fear of failure underlies his perfectionist behavior.
• The principles that underlie democracy include freedom and equality.
• Solid rock underlies the sandy soil in this area.
• What assumptions underlie your argument?
• A layer of clay underlies the garden bed.
• Cultural differences often underlie communication problems.