Common Words:
* ill: sick, unwell
* illegal: against the law
* illiterate: unable to read or write
* illogical: lacking logic or reason
* illusion: a false perception
* illustrate: to explain with pictures or examples
* illuminate: to light up; to make clear
* ill-tempered: easily angered
* illness: a state of being sick
* ill-fated: destined to have bad luck
Less Common Words:
* illiberal: narrow-minded
* illimitable: without limits
* illogicality: the state of being illogical
* illiquid: lacking in cash or easily convertible assets
* illegible: difficult to read
* illicit: illegal
* illicitly: in an illegal way
* illiteracy: the state of being unable to read or write
* illegitimate: not legal or valid
* illuminate: to make light of; to explain
* ill-starred: unlucky
* ill-timed: happening at an unsuitable time
Note: The prefix "il-" is often used to create negative or opposite meanings of words.