(in ‘psychology’, a stimulus is any object or event that elicits a sensory or behavioral response in an organism)
(in ‘perceptual psychology’, a ‘stimulus’ is an energy change (e.g., light or sound) which is registered by the senses (e.g., vision, hearing, taste, etc.) and constitutes the basis for ‘perception’)
(in ‘behavioral psychology’ (i.e., ‘classical’ and ‘operant’ conditioning), a stimulus constitutes the basis for ‘behavior’)
(in this context, a distinction is made between the distal stimulus (the external, perceived object) and the proximal stimulus (the stimulation of ‘sensory organs’))
(in ‘experimental psychology’, a stimulus is the event or object to which a response is measured)
(thus, not everything that is presented to participants qualifies as ‘stimulus’)
(for example, a cross mark at the center of a screen is not said to be a stimulus, because it merely serves to center participants’ gaze on the screen)
(also, it is uncommon to refer to longer events (e.g. the “Trier social stress test”) as a ‘stimulus’, even if a response to such an event is measured)