It is the product of nucleophilic addition of an aldehyde or ketone to an alcohol. 1 molecule of alcohol adds to 1 molecule of aldehyde or ketone to yield hemiacetal.

Formula: RCR'(OR")(OH). OR" has been supplied by the alcohol whereas R and R' originally belong to the aldehyde or ketone. If it is an aldehyde, then one of the two i.e. R or R' must be H.

PS: Hemi means half. Hemiketal name is no longer in use by IUPAC, it was used in the past to distinguish between aldehydes & ketones.