Tone Definition

Dictionary Definition:  Tone refers to the lightness or darkness of something. This could be a shade or how dark or light a colour appears. 

My Own Definition: The difference between the lightest and darkest areas of the environment.

Words to Describe 'Tone': Shades, Bright, Dark, Light, Dull, Colour, Black, White.

Four annotated pictures:

This picture is a prime example of what photographers mean by the term 'tone'. The colours have deliberately been removed so that we can see more easily what tones are evident in the picture. The people walking have been darkened so much that they resemble that of a 3 dimensional shadow.
  

Again, this picture is controversial because we're so used to seeing photos with their natural colours, yet this one has had it's colours removed so as to portray the variety in the brightness and darkness of the picture.

The fact that the colour has been removed in this photograph has worked to it's advantage, because the blackness of the rust on the railings is more prominent than it would have been if it was a photograph with a bright background.

This photograph can be described as controversial because it is actually quite confusing at first as to whether the colour has been taken away for the effect of tone or not, due to the actual colours being black and white in this environment, but judging by the clouds, I can see that this picture has in fact been effected in order to show tone.