Primary circular reactions are actions that an infant performs repeatedly, typically involving their own body, which occur during the sensorimotor stage of development as defined by Piaget. These reactions emerge between 1 and 4 months of age, when infants begin to intentionally repeat actions that are pleasurable or interesting, such as sucking their thumb or cooing. This stage highlights the growing cognitive awareness and ability to interact with their environment, laying the groundwork for further developmental progress.