A #1^o# or primary alcohol refers to the alcohol functional group (-OH) of a molecule being bonded to a carbon atom which is bonded to only one other carbon atom.
Explanation:
Examples:
ethanol
pentanol
A #2^o# or secondary alcohol refers to the alcohol functional group of a molecule being bonded to a carbon atom which is bonded to exactly two other carbon atoms.
A #3^o# or tertiary alcohol refers to the alcohol functional group of a molecule being bonded to a carbon atom which is bonded to exactly three other carbon atoms.