from The American Heritage® Dictionary of the English Language, 4th Edition
- conj. Except on the condition that; except under the circumstances that: "Exceptional talent does not always win its reward unless favored by exceptional circumstances” ( Mary Elizabeth Braddon).
- prep. Except for; except.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- conj. Except on a specified condition; if not.
from the GNU version of the Collaborative International Dictionary of English
- conj. Upon any less condition than (the fact or thing stated in the sentence or clause which follows); if not; supposing that not; if it be not; were it not that; except.
from The Century Dictionary and Cyclopedia
- If it be not that; if it be not the case that; were it not the fact that; if … not; supposing that … not.
- For fear that; in case; lest.
- [By omission of a verb, implied in the context, unless may have the force of ‘except,’ ‘but for’: as
- Synonyms Except, Unless. Except could once be used as a synonym for unless, but the words have now drawn entirely apart. Unless is only a conjunction; except is only a preposition. Except introduces an exception to a statement which is otherwise general: it may be followed by a clause when connection is made by a particle, as when, that, as, while, or especially another preposition: the omission of such connective makes the structure archaic. Unless introduces a clause, or the abbreviation of a clause, indicating a limitation or condition.
Middle English unlesse, alteration (influenced by un-, not) of onlesse : on, on; see on + lesse, less; see less.(American Heritage® Dictionary of the English Language, Fourth Edition)
onlesse, from on lesse, from on a less condition (on became un-) (Wiktionary)