diaeresis or dieresis /dī-ērˈi-sis or -erˈ/

1. A mark ( ¨ ) placed over a vowel, esp the second of two adjacent ones to show that it is to be pronounced separately, as in naïf
2. A pause or break where the end of the word coincides with the end of a foot (prosody)

• undersaturated
• under-sawyer
• under-sawyers
• undersay
• undersaye
• undersayes
• undersaying
• undersays
• under-school
• under-schools
• underscore
• underscored
• underscores
• underscoring
• underscrub
• underscrubs
• undersea
• under seal
• underseal
• undersealed
• under sealed orders
• undersealing
• undersealings
• underseals
• under-secretaries