If you are using the matrix control in Reporting Services, you may want to create a static heading over top of your column headers. For example:

You might first try putting a text box on the body above the matrix. But if you don't know how many columns you have, how can you be sure that it is centered? One trick is to use a static group for a header. To do this, right click on the matrix and add a new column group to the matrix. For the group expression, enter "=1". This will make it a static group. Make it the top group either by dragging it or rearranging it in the matrix properties dialog. In the header textbox, type your header and center it horizontally.

You can also use this technique for a static row header. The limitation there will be that Reporting Services doesn't support full text rotation so the best you can do if you want a vertical label will be to set the WritingMode property to "tb-rl".