A puff of air to the eye causes a reflexive blink. If you consistently precede the puff of air with a buzzer, eventually the buzzer alone will cause a blink. At this point, the buzzer is now the:

Conditioned stimulus  

Explanation:

In psychology, the conditioned stimulus is described as one of the different stimuli present in the theory of classical conditioning which was discovered by Ivan Pavlov. According to Pavlov, the conditioned stimulus is referred to as a stimulus that is being considered as formerly "neutral stimulus" that becomes connected with an unconditioned stimulus and therefore it triggers or give rise to a conditioned response.

According to the question, the buzzer is considered as the 'conditioned stimulus'.