I have recently started reading literature on 2 dimensional systems in Condensed matter. While reading, I frequently came across the word 'bulk'. Sometimes it referred to 2-D and sometimes to 3-D. I would like to know what does 'bulk' refer to, 2-D or 3-D?
In condensed matter "bulk" does not refer to the dimensionality of the problem but the location in the material. It refers to the volume of the crystal, as opposed to, e.g., surface effects.
Many organic conductors behave as 1D systems, yet you can talk about bulk properties.
Copper oxide superconductors have a 2D physics. However, often you will find discussions on whether a measurement (in particular ARPES and STM) is representative of bulk properties or of surface effects.
Strontium titanate is a 3D material, with insulating bulk properties. But when you deposit a lanthanum aluminate layer on it, you can create a superconductor on the interface alone.
Topological insulators, as tagged by your question, also have bulk characteristics and surface related properties that create the topologically protected states.
In summary, bulk refers to the volume of the material. You can think of bulk properties are those that an infinite material would have. It is not (necessarily) related to the dimensionality of the physics describing it. The importance of the terminology is to differentiate bulk properties from those arising from interface effets or from the symmetry breaking introduced by the "end" (the surface) of the infinite periodic array of atoms.