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A transformer that has two leads connected to the primary winding and three leads connected to the secondary winding, one of which is positioned in the middle, is identified as a center-tapped secondary transformer. The middle lead allows for the division of the secondary voltage into two equal parts, effectively creating two outputs that can be utilized for various applications, such as providing both positive and negative voltages relative to a common ground reference.

In this configuration, the center tap offers more flexibility in circuit design, allowing for dual power supply outputs from a single transformer. This is particularly useful in applications such as audio amplifiers or power supplies for op-amps, where symmetric voltages are needed.

The other types of transformers mentioned serve different purposes. A step-up transformer increases voltage from primary to secondary, while a step-down transformer reduces voltage. An isolation transformer is used primarily to electrically isolate circuits from each other, often for safety or noise reduction purposes, but does not include the center tap configuration referenced in the question. Thus, recognizing the characteristics of a center-tapped secondary transformer is crucial for understanding this topic in electronics.