- unknown function
- unknown number
- unknown parameter
- unknown quantity
- unlabelled
- unlabelled graph
- unlabelled tree
- unless
- unlesss (= precisely unless)
- unlike
- unlike fractions
- unlike sign
- unlike terms
- unlimited
- unlimited abstraction principle
- unlimited covering manifold
- unlimited decimal
- unlimited function
- unlimited variation
- unlink
- unlinked
- unlinking
- unloaded standby
- unmeasurable
- unmixed