What is a defining characteristic of a laser beam?

A defining characteristic of a laser beam is that it is tightly focused, or collimated. This means that the beam travels in parallel rays with minimal spreading over distance, which allows it to maintain its intensity and coherence. This property is crucial for many applications of lasers, as it enables precise targeting and effective energy delivery to a specific area.

The collimation of laser light results from the stimulated emission process that occurs within the laser medium, creating light that is coherent and monochromatic. This coherence is what differentiates laser light from other light sources, which tend to emit light in many directions and over a range of wavelengths.

In contrast, while a laser beam can indeed vary in size depending on the design and application of the laser system, the key defining characteristic lies in its ability to remain focused over long distances. Multiple wavelengths would indicate more spread and less coherence, which is not a defining feature of laser light. Additionally, omnidirectional light describes light that radiates in all directions, which is not applicable to the focused, directional nature of a laser beam.