1. Gentle utterance use to call someone's attention
   - ahem, hem 
2. Exclamation expressing doubt, deliberation, dissent, or hesitation
   - hum, ahem

Encyclopedia: Hmm

• Universal Product Code
• universal proposition
• universal quantifier
• universal resource locator
• universal set
• universal solvent
• universal suffrage
• universal time
• universal veil
• universalise
• universalism
• universalist
• universalistic
• universality
• universalize
• universally
• universe
• universe of discourse
• university
• university extension
• University of California at Berkeley
• University of Chicago
• University of Michigan
• University of Nebraska
• University of North Carolina