Which adjective describes something that is beginning to happen or develop?

The adjective that describes something that is beginning to happen or develop is "incipient." This term is often used to refer to initial stages or early signs of a condition, trend, or phenomenon. For example, one might talk about an incipient illness, meaning that the illness has just started to manifest but is not yet fully developed.

In contrast, "nebulous" refers to something that is vague or unclear, "intrepid" characterizes someone who is fearless or adventurous, and "aberrant" describes something that deviates from what is normal or expected. These terms do not capture the idea of something that is just starting or emerging, which is why they do not fit the definition provided in the question.