Homoplasmy is the presence of a mutation affecting all of the mitochondrial DNA (mtDNA) copies in a mammalian cell or chloroplast DNA in a plant cell. However, when a mutation occurs, it only affects some of the mitochondria, but not all of them known as heteroplasmy. Since there are hundreds or even thousands of mtDNA copies in every eukaryotic cell, mutations may either be present in all copies (homoplasmy) or affect only a fraction of them (heteroplasmy). Homoplasmy may also refer to the presence of mutation, or insertion of a foreign gene, into all of the plant plastid organelles DNA (e.g. all chloroplasts). Homoplasmy, more generally, is the state whereby all non-nuclear genomes are the same, whether wild type or mutated.
A mutation load needs to be present before disease occurs, which is called the "Threshold effect." This load is generally 80% or above mutated mitochondria in a given cell. Mitotic segregation may affect the mutant mtDNA among the daughter cells. Bottom line, in order to get mitochondrial disease you need to have certain % (usually above 80%) of mutant mtDNA to show the phenotype.