Actually, a particle of light is always a photon. In the context of quantum mechanics, light is understood to consist of individual packets or quanta of energy called photons. Photons are fundamental particles, meaning they cannot be broken down into smaller components. Therefore, a particle of light is always a photon.
In terms of the definition of a particle, it can be a small part of something, but in the case of a photon, it is considered a particle in the sense that it has both wave-like and particle-like behavior. This is known as wave-particle duality, which is a fundamental concept in quantum mechanics. When photons are observed or measured, they exhibit particle-like behavior, such as being localized in a specific position. However, when they are not observed, they behave like waves, exhibiting interference patterns and spreading out over space.