Plagiarism, as defined by the Oxford English Dictionary, is “the action or practice of taking someone else's work, idea, etc., and passing it off as one's own.”
This includes copying and pasting text from other sources and inserting them into your own work, or even just rewording text from another source. Anything that does not reflect your own thoughts or ideas, with incorrect attributions, or a lack thereof, is an act of plagiarism.