jquery, Showing a hidden item with a fadeIn

Just hide the element initially, ether with .hide() or style="display: none;" (or display: none; in the stylesheet). Then, just call .fadeIn(), like this:

$("#elementID").fadeIn();

The .fadeIn() call automatically removes the display: none when it fades the opacity to 100%, it won't remove visibility: hidden; so don't use this, or you'll have to remove it manually.

Use fadeIn():

$('#hiddendiv').fadeIn();

You may change the duration of the fadein:

$('#hiddendiv').fadeIn(1000); // 1000 ms