2D-otsu algorithm in Digital image processing
The Otsu algorithm most responders are referring to, is the 1D algorithm indeed, exactly for the reason mentioned above. The main drawback is that it does not work very well in images with low s/N ratio or low contrast. Also, when you only have a small object with a big background, Otsu will incorrectly select background pixels. 

The 2D Otsu provides a solution to this. I don't know the maths involved, but it performs '2D histogram projection', I found it was well documented in: 
J. Zhang, J. Hu, 2008 International Conference on Computer Science and Software Engineering
From what I understand it projects the histograms in such way and then applies some filters to it so that the peaks relating to the 'object' will appear more profound in the resulting histogram, enabling better segmentation. 

Both algorithms can be applied in 2D and 3D ( to add some more confusion).