What is a bulk phase transition? I have been able to google "bulk phase transition" and get plenty of results that verify that something called a bulk phase transition exists, however, I cannot seem to find a precise definition of what exactly a bulk phase transition is.  Could someone please help me with a definition of bulk phase transition? I would very much appreciate it.
 A: I think the word bulk transition is used in several (related) contexts. The first is the following: Consider a system with a first order phase transition, governed by some partition function in $d$ dimensions. Impose boundary conditions that create a $d-1$ dimensional interface between the two phases. Construct an effective partition function (in $d-1$ dimensions) for the interface. You can now ask whether the roughening transition for the $d-1$ dimensional interface occurs at the same temperature as the $d$ dimensional bulk transition. 
A (related) usage of the term bulk transition is in lattice field theory, usually in the context of first order transitions of lattice gauge theories at strong coupling. A weak coupling transition of a $d+1$ dimensional lattice model is related to a thermal phase transition of a continuum field theory in $d$ spatial dimensions. But this is not the case for a strong coupling transition -- there is no continuum limit, and therefore no thermal interpretation of the boundary conditions. The transition is merely a bulk transitions of the $d+1$ dimensional lattice model. 
A: I think you should not put the question in such a way: "What exactly a bulk phase transition is?" concerning such matters. It is because it is not a precisely defined scientific term. It is rather an explanatory term that people may use to indicate different things. In such a case they, of course, should explain, what do they mean.
 I am one of those who often uses the term "bulk phase transition". I use it to stress the difference between the transition taking place in the whole (or most of) the material bulk and the one only taking place locally. The local case may take place say, when the transition into the daughter phase takes place say, in the vicinity of a surface or of some defect, like say, a dislocation or an inclusion, while the bulk of the body stays in the mother phase. 
  I hope it helps. 
A: I would say that a bulk phase transition can appear in a liquid or a solid. In a liquid however the differences would be less pronounced since the difference between the bulk and the surface is only some interface energies. In solid state phase transitions (normally reffered to as transformations) the strain energy also depicts a major contribution to the Gibbs free energy (analogy of pressure in gases). Therefore, since these stresses are lower near unconstrained surfaces, one could use the term bulk phase transition to restrict his representative volume element fully to a bulk, e.g. by applying periodic boundary conditions
