Irony isn't a concept that lends itself to a trivial definition. That particular dictionary definition you cite isn't completely wrong, but it does suffer greatly from being overly terse.

The Merriam-Webster definition is much better: http://www.merriam-webster.com/dictionary/irony

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Furthermore, dictionary definitions aren't two way streets, not everything that fits a description therefore matches the word. Imitate, the verb, can mean to make a copy of. "Please wait while I imitate the receipt for you."