What is MHC haplotype?
A haplotype is commonly defined as 


  ...a group of genes within an organism that was inherited together from a single parent. -- Nature Scitable


This definition of haplotype applies whether it is just a few genes or all of the genes on an entire chromosome$^1$. In the case of the major histocompatibility complex, it is a group of more than 200 genes on chromosome 6 (Janeway et al. 2001). The set of alleles$^1$ inherited together from one parent form the MHC haplotype. Thus, one offspring will inherit two different MHC haplotypes, one from each parent. The different haplotypes with so many genes helps keep genetic variation in the overall complex, which improves the immune response ability of the organism.  

The term is derived from haploid, which means half of the normal set of chromosomes. Humans for example are diploid, meaning it has two copies of each chromosome, one inherited from the mother (maternal) and one inherited from the father (paternal). So, when you inherit just one half of the crhomosome pair, you are inheriting a haplotype.

For general reference, it's important to note that haplotype was not always used to reference a group of two or more genes inherited together. It was commonly used to as a term to represent a sequence of nucleotides together on the same chromosome, even if only part of a single gene. For example, Bowen et al. (1995) used mitochondrial haplotypes to study the migration of loggerhead turtles across the Pacific Ocean. They used 350 nucleotides of the mitochondrial control region. The control region is over 1000 nucleotides in size but Bowen and colleagues only used 350 nucleotides from part of the control region.  This use was very common in the 1990s and early 2000s before it became easier and less expensive to sequence entire genomes, or at least many genes.

Citaton

Bowen, B.W. et al. 1995. Trans-Pacific migrations of the loggerhead turtle (*Caretta caretta) demonstrated with mitochondrial DNA markers. Proceedings of the National Academy of Sciences USA 92: 3731-3734.

Janeway, Jr., C.A. et al. 2001. Immunobiology, 5th edition. Garland Science, New York.

Footnote


More accurately, it would be the set of alleles at each gene locus (gene location) inherited together. The same genes are inherited from either parent, but the alleles (variations of the gene) may be different between the two parents.