The methyl group is a functional group with the formal unit CH2. The simplest chemical compound having a methyl group is methylene ( CH2), a highly reactive carbene.

According to IUPAC nomenclature, the prefix methylene substituents for a double-bonded ( = CH2), a divalent group ( -CH2-) and the name of chain and ring members are used. A special case is the name given to the highly reactive carbene as " methyl molecule" represents the group = CH2 is now referred to as methylidene.

When activated methylene compounds are referred to substances in which the carbon- hydrogen bond of a methyl group is particularly acidic by electron-withdrawing substituents, ie for example malonic esters and β -keto esters. These esters are CH-acidic compounds and are often used in the Knoevenagel reaction which employed a Michael addition, or similar reactions, when the starting materials.

The term methylene (from Greek methy = alcohol. Beverage and Hyle = wood) was coined in 1834 by Jean Baptiste Dumas and Eugène- Melchior Peligot and refers to the former name of methanol as " wood alcohol ", the dehydrated form is formally the methyl group.

Methylene chloride ( CH2Cl2) is a commonly used name for dichloromethane.