Allele: A variant of a gene. 

Autosome: Any chromosome other than a sex chromosome.

Carry: This means that the rabbit is hiding an allele other than the one that is visible in the phenotype. In regards to color genetics, a rabbit can only carry an allele for a variety that is recessive to its variety. A rabbit that is heterozygous is said to 'carry' a more recessive allele.

Chromosomes: The genetic structure of cells that reproduces itself. It contains the DNA. Domestic rabbits have 44 chromosomes (or 22 pairs of chromosomes), cottontails have 42, and hares have 48.

Dominant: A dominant allele is one that is 'stronger' than another allele. A dominant allele always shows up in the phenotype if it is present. For example, the gene for Black is dominant to the allele for Blue. Therefore if a rabbit had one allele for Black and one allele for Blue, the rabbit would appear as a Black.

DNA (deoxyribonucleic acid): The molecule that encodes genetic information. It is what tells all living beings how to look, and it can influence or control other things such as temperament, resistance to disease, etc.

Genotype: This tells us what the rabbit is genetically. It tells us what the rabbit carries, what alleles it has, etc. It is expressed as a series of letters. For example, Chestnut is expressed as A_ B_ C_ D_ E_ enen. The blank spots just mean that we are not sure about the other allele in that locus, and the rabbit could be carrying other genes in those spots.

Heterozygous: This means the rabbit is not pure for the allele. It has one of one allele, and it carries another.

Homozygous: This means the rabbit is pure for that allele. It has two of those alleles, and does not carry any other allele.
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Locus (plural Loci): This is the position of the gene on the chromosome. It is the set or series that a particular gene is in. Each loci is given a specific letter name. Examples are the A locus, the B locus, the C locus, etc. Genes from one loci cannot be dominant or recessive to genes from another loci, because they do not actually affect each other. There can only be two genes per loci. One gene per locus is donated from each parent, filling the two "slots" that each locus has.
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Modifier: A type of allele that is directly linked with another allele and modifiers the effect of the allele it is linked with. It cannot make the characteristic that it is linked to, it just alters it. An example is rufus modifiers. A rabbit with a higher number of rufus modifiers will have deeper, darker red coloring. A rabbit with a lower number of rufus modifiers will have lighter, duller red color.

Mutation: A change in the DNA that can be inherited. All varieties except Chestnut Agouti are mutations of the original wild color of rabbits, which is Chestnut Agouti. All fur types other than normal are mutations. Lopped ears are caused by a mutation.

Phenotype: This is what we see when we look at a rabbit. If we say a rabbit is white with black spots, we are describing its phenotype.

Recessive: A recessive allele is one that is 'weaker' than another allele. A recessive allele's effect can hide behind a dominant allele's effect. For example, the allele for Blue is recessive to the allele for Black. Therefore if a rabbit had one allele for Black and one allele for Blue, the rabbit would appear as a Black instead of a Blue. The only way for the rabbit to appear as a Blue would be for both alleles to be the one for Blue.

Sex Chromosome: One of the two chromosomes that determine the rabbit's sex (buck or doe). Normal females have two X chromosomes, and normal males have one X chromosome and one Y chromosome.

For a very comprehensive list of genetic terms, please go here. Be forewarned, it is NOT for beginners, but it is very informative.