I'll keep it short but please read through!
Gravitation's $1/r$ potential means everything in the solar system is pulling on everything all the time.
Gravitational slingshot maneuvers are normally though of as passing fairly close to a massive object producing a large deflection and change in velocity, but there's no cut-off; all bodies are affecting a spacecraft orbits and if you don't take several into account your calculated trajectory will not reflect where your spacecraft ends up.
So there probably isn't a universally agreed-upon objective mathematical test do say if a particular solar-system's body's effect on a trajectory is above or below "slingshot threshold" but there may exist a particularly weak one that was still a deliberate gravitational slingshot maneuver.
Question: What was the smallest intentional, acknowledged slingshot maneuver? Does one stand out as having the lowest delta-v or smallest deflection angle, or perhaps was the least beneficial, and yet was still acknowledged as a deliberate gravitational assist?
Discussion below this question got me started on this.