Select the most appropriate meaning of the given idiom.

A pipe dream

An illusory plan

The most appropriate meaning of the idiom "pipe dream" is an illusory plan.

The other options are incorrect.

• An illusory plan is a plan that is unlikely to happen.
• A secret is something that is kept hidden from others.
• A realistic dream is a dream that is possible to achieve.
• A clever strategy is a plan that is likely to succeed.

The idiom "pipe dream" comes from the idea of a pipe being used to smoke tobacco, which is a habit that is often associated with unrealistic thinking. In the same way, a pipe dream is a plan that is unlikely to happen, or a dream that is not based in reality. For example, you might say that winning the lottery is a pipe dream.