The functional group that characterizes alcohols is known as the hydroxyl group, represented as $-OH$.

Alcohols are a class of organic compounds defined by the presence of one or more hydroxyl groups ($-OH$) bonded to carbon atoms. This hydroxyl group is responsible for the unique physical and chemical properties of alcohols. The simplest alcohol is methanol, with the chemical formula $CH_3OH$, where the hydroxyl group is attached to a single carbon atom.

The hydroxyl group consists of an oxygen atom covalently bonded to a hydrogen atom. This bond imparts polarity to alcohols, creating a molecule with a positive end and a negative end. This polarity enables alcohols to form hydrogen bonds with other molecules, including water, which explains their general solubility in water.

The presence of the hydroxyl group also significantly influences the physical properties of alcohols. For example, alcohols typically exhibit higher boiling points compared to hydrocarbons or ethers of similar molecular size. This phenomenon occurs because the hydrogen bonds between alcohol molecules require more energy to break than the weaker van der Waals forces present in other organic compounds.

Regarding chemical reactivity, the hydroxyl group can engage in various chemical reactions. For instance, alcohols can undergo dehydration to yield alkenes, or they can be oxidized to produce aldehydes, ketones, or carboxylic acids, depending on the specific alcohol involved and the reaction conditions.

In conclusion, the hydroxyl group is the fundamental functional group of alcohols, shaping their physical properties, solubility, and reactivity. A thorough understanding of this functional group is essential for comprehending the behavior of alcohols in chemical reactions.