| Term | Definition |
| monastery | religious house where one goes to be alone, away |
| monotone | having one sound or tone |
| monogamy | having one spouse |
| monopoly | control of sales by one person or company |
| monocle | an eyeglass for one eye |
| monolith | one single, large stone or monument |
| union | act of combining separate items into one |
| unique | one of a kind |
| monophobia | fear of being alone |
| unison | sung with one voice |
| bicuspid | a two-pointed tooth located in the side of the jaw |
| bicep | a muscle having two origins |
| bifocal | having two centers of vision |
| dialogue | a conversation between two people |
| duel | combat between two people or armies |
| biped | a two-footed creature |
| bicentennial | occurring every two hundred years |
| bilingual | speaking two languages |
| duet | music for two performers |
| duplicity | double-dealing, trickery |