A histamine antagonist is an agent which serves to inhibit the release or action of histamine.
Clinical: H1- and H2-receptor antagonists
In common use, the term antihistamine refers only to H1-receptor antagonists, also known as H1-antihistamines. It has been discovered that these H1-antihistamines are actually inverse agonists at the histamine H1-receptor, rather than antagonists per se. 
Clinically-relevant histamine H2-receptors are found principally in the parietal cells of the gastric mucosa. H2-receptor "antagonists" are also inverse agonists, rather than true antagonists; and are used to reduce the secretion of gastric acid. Examples include cimetidine, ranitidine, and famotidine.
Experimental: H3- and H4-receptor antagonists
These are experimental agents and do not yet have a defined clinical use.
Inhibitors of histamine release
These agents (mast cell stabilizers) appear to stabilise the mast cells to prevent degranulation and mediator release.
Other agents with antihistaminergic activity
- Leurs R, Church MK, Taglialatela M (2002). "H1-antihistamines: inverse agonism, anti-inflammatory actions and cardiac effects". Clin Exp Allergy. 32 (4): 489–98. PMID 11972592.