What is the defining characteristic of a missile with a semi-active seeker?

The defining characteristic of a missile with a semi-active seeker is that it requires an external illumination source. This type of seeker relies on a radar source that illuminates the target, allowing the missile to home in on the reflected signals. When the missile is fired, it does not generate its own radar signals; instead, it detects the radar energy reflected off the target, which is typically provided by a radar system on a platform such as a ship or aircraft.

In contrast, missiles that use their own radar (as seen in active seekers) are entirely self-sufficient once launched. Similarly, targeting emissions from an enemy vessel relates to the passive guidance system where the missile detects the heat or other signatures from a target without needing an external source. Lastly, using images for guidance pertains to imaging seekers, which is an entirely different technology focused on visual recognition rather than relying on radar illumination. Thus, having an external illumination source is what distinctly characterizes a semi-active seeker missile.