Plagiarism is the passing off of someone else’s work as one’s own, and is cheating.
Plagiarise - "Take and use as one’s own (the thoughts, writings, inventions etc., of another person); copy (literary work, ideas etc.) improperly or without acknowledgement; pass off the thoughts, work, etc., of (another person) as one's own."
From: The New Shorter Oxford English Dictionary.
View fact sheet on plagiarism from Elsevier.
To avoid plagiarism you must give credit whenever you:
Using an image, video or anything else you find online in your work without providing a correct citation is plagiarism.
The only source you can use in an essay without crediting it, is information that is considered common knowledge and as a result not attributable to one source.
Common knowledge is information generally known to a knowledgeable person, such as commonly known facts and dates.
Examples of common knowledge are:
GMIT uses URKUND, a leading academic plagiarism detector, to check students' work for plagiarism by comparing it against a large academic database.
URKUND is an automatic text-recognition system made for detecting, preventing and handling plagiarism.
URKUND shows lecturers’ how much of a student's work matches content from their databases; so they can understand how much of the work is original.