Arousal refers to a state of alertness and mental and physical activation of a human. It is an activation in any of the body's systems (e.g., activation of the sympathetic nervous system or the cerebral cortex). 

Arousal as a general term has resisted definition, because sometimes one system is activated while another is not.

Other /More definition:
Arousal refers to a phase in the sexual response cycle, psychological experience of arousal and pleasure as well as physiological changes, such as the tensing of muscles and enlargement of blood vessels and tissues. It is a degree of physiological excitation, responsivity, and readiness for action, relative to a baseline; a physiological reaction, including faster heartbeat and faster or heavier breathing, linked to most conscious emotions Arousal is also called the Excitement phase

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