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They are not synonyms.
display:none removes the element from the normal flow of the page, allowing other elements to fill in.
visibility:hidden leaves the element in the normal flow of the page such that is still occupies space.
Imagine you are in line for a ride at an amusement park and someone in the line gets so rowdy that security plucks them from the line. Everyone in line will then move forward one position to fill the now empty slot. This is like display:none.
Contrast this with the similar situation, but that someone in front of you puts on an invisibility cloak. While viewing the line, it will look like there is an empty space, but people can't really fill that empty looking space because someone is still there. This is like visibility:hidden.
display:none will hide the element and collapse the space is was taking up, whereas visibility:hidden will hide the element and preserve the elements space. display:none also effects some of the properties available from javascript in older versions of IE and Safari.
If visibility property set to "hidden", the browser will still take space on the page for the content even though it's invisible.
But when we set an object to "display:none", the browser does not allocate space on the page for its content.
Example:
<div style="display:none">
Content not display on screen and even space not taken.
</div>
<div style="visibility:hidden">
Content not display on screen but it will take space on screen.
</div>