In genetics, penetrance refers to the proportion of individuals with a particular gene that express that gene phenotypically. Recall that an organism's phenotype is its physical properties, which are in part caused by its genes. So in other words, if 95% of all the people with the Huntington's gene go on to clinically develop Huntinton's disease, the gene can be said to have 95% penetrance.
I'm not convinced that penetrance is as watertight a concept as it should be. Take, for instance, phenylketonuria, an autosomal recessive disease characterised by mental retardation caused by an inability to metabolise the amino acid phenylalanine. Virtually every single child who is homozygous for this deleterious gene will go on to a lifetime of mental retardation and seizures if fed a normal diet. Yet, if it is fed a special diet that lacks this amino acid, the child will be symptom-free. So what is the penetrance in this case? 100% or 0%?
There are other problems with penetrance too, but it has entrenched itself as a useful tool (in abstraction at least) within the subject of genetics. It should not be confused with expressivity, which measures the degree of expression of a genetically-determined trait. Penetrance is used when the phenomenon is a digital all-or-nothing (i.e. you either have Huntington's disease or you don't; you can't 'half' have it).