forbidden by law or statute.
contrary to or forbidden by official rules, regulations, etc.: The referee ruled that it was an illegal forward pass.
Informal. illegal alien.

1620–30; < Medieval Latin illēgālis. See il-2, legal

illegally, adverb

illegal, illicit (see synonym study at the current entry).

1. unlawful; illegitimate; illicit; unlicensed. Illegal, unlawful, illegitimate, illicit, criminal can all describe actions not in accord with law. Illegal refers most specifically to violations of statutes or, in organized athletics, codified rules: an illegal seizure of property; an illegal block (in football ). Unlawful means not sanctioned by or according to law: an unlawful claim to the inheritance; to take unlawful advantage of the trading situation. Illegitimate means lacking legal or traditional right or rights: an illegitimate child; illegitimate use of privileged knowledge. Illicit which originally meant simply “not permitted,” now most often applies to matters regulated by law with specific emphasis on the way things are carried out: illicit conversion of property; an illicit attempt to control the market. Criminal most often refers to violation of the statutes of penal as opposed to civil law. All felonies are criminal as are all crimes sometimes punishable by death such as murder, arson, and kidnapping: a criminal act. Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
illegal (ɪˈliːɡəl)
1.  forbidden by law; unlawful; illicit
2.  unauthorized or prohibited by a code of official or accepted rules
3.  a person who has entered or attempted to enter a country illegally

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

1626, from M.L. illegalis, from L. in- "not" (with assimilation of -n- to the following consonant) + legalis (see legal). Term illegal immigrant first recorded 1939 in ref. to Jews entering Palestine without authorization during the British mandate.
Online Etymology Dictionary, © 2010 Douglas Harper
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Example sentences
Loggers also destroy silky habitat when they cut down, illegally, valuable
  rosewood trees.
We took out tens of thousands of cars that used to park illegally on the
They were not listed on the manifest and thus imported illegally, but all the
  same they were only farm machinery.
The clear implication being that these fossils must therefore have been
  collected or exported illegally.
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