In organic chemistry and organometallic chemistry, carbon–hydrogen bond activation ( activation) is a type of organic reaction in which a carbon–hydrogen bond is cleaved and replaced with a  bond (X ≠ H is typically a main group element, like carbon, oxygen, or nitrogen).  Some authors further restrict the term C–H activation to reactions in which a C–H bond, one that is typically considered to be "unreactive", interacts with a transition metal center M, resulting in its cleavage and the generation of an organometallic species with an M–C bond. The intermediate of this step (sometimes known as the  activation step) could then undergo subsequent reactions with other reagents, either in situ or in a separate step, to produce the functionalized product. 
The alternative term  functionalization is used to describe any reaction that converts a relatively inert  bond into a  bond, irrespective of the reaction mechanism (or with an agnostic attitude towards it).  In particular, this definition does not require the cleaved C–H bond to initially interact with the transition metal in the reaction mechanism.  This broader definition encompasses all reactions that would fall under the restricted definition of C–H activation given above.  However, it also includes iron-catalyzed alkane C–H hydroxylation reactions that proceed through the oxygen rebound mechanism (e.g. cytochrome P450 enzymes and their synthetic analogues), in which a metal–carbon bond is not believed to be involved.  Likewise, the ligand-based reactivity of many metal carbene species with hydrocarbons in which the carbene carbon inserts into a C–H bond, again without interaction of the hydrocarbon C–H bond with the metal, also falls under this category.  Often, when authors make the distinction between C–H functionalization and  activation, they will restrict the latter to the narrow sense.