Autosomal dominance is a type of genetic inheritance involving a gene on any chromosome that is not a sex chromosome. A person who carries that gene on one allele will express (experience effects from) that gene. An autosomal dominant gene can be inherited from a single affected parent; both sons and daughters have an equal chance (50%) of inheriting the gene.
If both parents carry the same autosomal dominant gene, the effects of that gene on their offspring may be magnified. An example of this is the gene that causes achondroplasia (dwarfism). The achondroplasia gene causes stunted bone growth; if a person inherits one achondroplasia gene and one normal gene, he or she develops as a dwarf. However, if two people with dwarfism produce a fetus with two copies of the achondroplasia gene, the fetus almost invariably dies in the womb due to fetus' skeleton failing to grow into anything capable of supporting life.
Some of this writeup is based on work I did for the science dictionary at http://biotech.icmb.utexas.edu/