Ortho acids are the most hydroxylated acid in a set of oxoacids, such as orthophosphoric acid. In general, when an element can form several oxoacids, these are called ortho-, meta- or pyroacids, with the ortho acid being the most hydrated of the three.
Organic ortho acids resemble hydrated carboxylic acids having the general structure RC(OH)3; orthocarbonic acid (C(OH)4) is generally considered a member. These compounds are unstable, however their corresponding orthoesters (RC(OR’)3) have reasonable stability and possess a rich chemistry. The equivalent nitrogen compounds are also hypothetical with guanidines being the closest real-world analogues.
References
Ortho acid Wikipedia(Text) CC BY-SA