Determine the meaning of the word vigilante,

A group of people who undertake law enforcement in their community without legal authority.   
   
How I remember, is Oliver Queen from the show Arrow. They called him a vigilante in a green hood :)

The definition of vigilante basically means someone who doesn't work with the law or by the laws and tries to make their own decision of justice, like the hero green arrow, he is a vigilante because he doesn't work with the law and doesn't go by the rules but instead makes his own

It is the poet alfred, lord tennyson himself