A type of blood group/blood type classification. People with Bombay phenotype are blood type Oh and are genotypically homozygous for the silent h allele (hh) of the H substance.
The rare individuals with Bombay phenotype do not express H substance on their red blood cells and therefore do not bind A or B antigens. Instead, they produce antibodies to H substance (which is present on all red cells except those of hh phenotype) as well as to both A and B antigens and are therefore compatible only with other hh donors.
Practically speaking, individuals with Bombay phenotype blood groups are only compatible blood donors for other Bombay phenotype individuals. Given that this condition is very rare to begin with, if you have this blood group and you need an urgent blood transfusion, you might be simply out of luck, as it would be pretty unlikely that any blood bank would have any in stock.