A ketal is a compound that has the following general structural formula.

R¹ = alkyl, aryl
R² = alkyl, aryl
R³, R⁴ = alkyl (In most ketals, R³ = R⁴; cyclic ketals are an exception.)

eg:

The functional group 1 in an organic molecule is called the ketal group; the carbon atom bearing the two oxygen atoms is the ketal carbon.

see also cyclic ketal