Hardware: For a computer system, all its physical components, as distinguished from the programs and data that are manipulated by the computer.

Homology: Two or more biological species, systems or molecules that share a common evolutionary ancestor. (general) Two or more gene or protein sequences that share a significant degree of similarity, typically measured by the amount of identity (in the case of DNA), or conservative replacements (in the case of protein), that they register along their lengths. Sequence "homology" searches are typically performed with a query DNA or protein sequence to identify known genes or gene products that share significant similarity and hence might inform on the ancestry, heritage and possible function of the query gene.

Homolog: One of a pair of chromosomes, in which one is obtained from the organism's maternal parent and the other from the paternal parent; found in diploid cells.

Homologous genes: Genes that share a common evolutionary ancestor.

Homologous protein: A protein that is related to another by common evolutionary history.

Homology domain: A region in a protein sequence with similarity to an otherwise unrelated protein. This term should be used only if the region is of a size sufficient to form a domain.

Hidden Markov Model (HMM): An extension of a Markov model, in which a state has a probability of emitting some output; thus, states may be "hidden."

High-scoring segment pair (HSP): Local alignments with no gaps that achieve one of the highest alignment scores in a given search.

Home page: A document on the World Wide Web that acts as a front page or point of welcome to a collection of documents that may introduce an individual, organization, or point of interest.

Hydrophilic: Liking water. An amino acid residue with a polar side chain, such as lysine or aspartic acid, that can form a hydrogen bond with water.

Hydrophobic: Fearing water. An amino acid residue with an aliphatic or aromatic side chain, such as phenylalanine, that cannot bond with water.

Hyperlink: A connection between hypertext documents that allows a reader to trace concepts appearing in one document to related occurrence in other documents.

Hypertext Markup Language (HTML): A standard, text-based language used to create hypertext documents to be viewed on the World Wide Web.

Hypothesis: An idea that can be experimentally tested; an idea with the lowest level of confidence.