- Under the table
- Under-age
- Under-production
- Undercharge
- Undercloth
- Underclothes
- Undercurrent
- Undergone
- Underground
- Underhand
- Undermanned
- Undermine
- Underrate
- Underscore
- Undersell
- Underside
- Undersirous
- Understate
- Undertaken
- Undertone
- Undervaluation
- Undervalue
- Underwater
- Underway
- Underwear
- Underweight
- Underwhelming
- Underworld
- Underwrite
- Undeservedly
- Undeveloped foetus
- Undiminished
- Undisclosed
- Undistorted
- Undo
- Undone
- Unencumbered
- Unending
- Unequivocal
- Uneven
- Unevenly
- Unevenness
- Unfair
- Universalist
- Uncurl
- U-boat
- Uber
- Ubermensch
- Ubiquity
- Uddered
- Ugliness
- Ugly
- Ugly woman
- Ugologist
- Ulcerated
- Ulceration
- Ullage
- Ulterior
- Ultimate
- Ultimatum
- Ultimo
- Ultra violet
- Ultraviolet rays
- Umbrage
- Unable
- Unaccompanied
- Unaccountable
- Unaided
- Unambiguous
- Unannounced
- Unanswerable
- Unanticipated
- Unarm
- Unavailingly
- Unawares
- Unbar
- Unbearbleness
- Unbeatable
- Unbeknownst
- Unbend
- Unbiased
- Unblossomed
- Unbolt
- Unbowed
- Unburden
- Unbutton
- Uncertain
- Uncertainty
- Unchallengeable
- Unchaperoned
- Uncivilised
- Uncle sam
- Unclear
- Unclear fission
- Unclouded
- Unco-ordinated
- Uncomely
- Uncomfortable
- Uncomprehending
- Unconditionally