Hemiacetals and hemiketals are compounds that are derived from aldehyde

Aldehyde

An aldehyde is an organic compound containing a formyl group. This functional group, with the structure R-CHO, consists of a carbonyl center bonded to hydrogen and an R group....

s and ketone

Ketone

In organic chemistry, a ketone is an organic compound with the structure RCR', where R and R' can be a variety of atoms and groups of atoms. It features a carbonyl group bonded to two other carbon atoms. Many ketones are known and many are of great importance in industry and in biology...

s respectively. The Greek word hèmi means half. These compounds are formed by formal addition of an alcohol

Alcohol

In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....

 to the carbonyl

Carbonyl

In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups....

 group.

When the alcohol group is replaced by a second alkoxy group

Alkoxy group

In chemistry, the alkoxy group is an alkyl group singular bonded to oxygen thus: R—O. The range of alkoxy groups is great, the simplest being methoxy . An ethoxy group is found in the organic compound phenetol, C6H5OCH2CH3 which is also known as ethoxy benzene...

, an acetal

Acetal

An acetal is a molecule with two single-bonded oxygen atoms attached to the same carbon atom.Traditional usages distinguish ketals from acetals...

 or a ketal, respectively, is formed.
The general formula of a hemiacetal is R¹R²C(OH)OR, where R¹ or R² is often hydrogen and R (bonded to O) is not hydrogen.

While in the IUPAC definition of a hemiacetal R¹ or R² may or may not be a hydrogen, in a hemiketal none of the R-groups is an H.