Final answer:
The term for the initial phase of conditioning when the CS and US are paired is called acquisition in classical conditioning.
Step-by-step explanation:
In classical conditioning, the initial phase of conditioning when the conditioned stimulus (CS) and unconditioned stimulus (US) are paired is called the acquisition period. During this period, the neutral stimulus (CS) begins to elicit the conditioned response (CR), and eventually becomes a conditioned stimulus capable of eliciting the CR on its own.
For example, in Pavlov's experiments with dogs, the tone (CS) and meat powder (US) were paired. Over time, the tone alone began to elicit salivation (CR) from the dogs, indicating that the acquisition of the conditioned response had taken place.
Timing is important for conditioning to occur, and usually, there should be a brief interval between the presentation of the CS and the US. With different types of conditioning, this interval can range from a few seconds to several hours.