While Goku famously unlocked the transformation of Super Saiyan during his iconic battle with Frieza on Planet Namek, he actually wasn’t the first Dragon Ball character to be considered a ‘Super Saiyan’ in continuity as that honor actually goes to his son, Gohan.
In Dragon Ball’s Frieza Saga, the villainous Lord Frieza invades Planet Namek with the full-force of his army in an attempt to collect the planet’s Dragon Balls. Frieza hoped to use the wish-granting magic of the Dragon Balls to become immortal so that he could rule over the universe unchallenged until the end of time. However, when the Z Fighters got wind of Frieza’s evil plan, they rushed to Namek to stop him–which is something that most of them would quickly regret. Frieza and his army wiped the floor with the Z Fighters and the heroes’ only saving grace (aside from Goku) was Vegeta who, at that point, was also a deadly villain. Once Frieza decided he was done letting the Z Fighters get in the way of his plans, the villain incapacitated the entire team and carried out the murders of both Vegeta and Krillin. While Vegeta’s death was shocking, it was Krillin’s that left a greater impact as Frieza killed the Z Fighter right in front of Goku, and the rage Goku felt at that moment was the spark he needed to unlock the legendary power of Super Saiyan.
In Dragon Ball chapter 204 by Akira Toriyama, Piccolo and Goku are successful in killing Raditz and saving Gohan, but at the cost of Goku sacrificing his own life in the process. As Raditz lay dying, he tells Piccolo and the other Z Fighters that he was just the first of his kind to come to Earth, and that two more Saiyans–Nappa and Vegeta–were on their way there at that very moment (they were much more powerful than Raditz). At that point, the book cuts to Vegeta and Nappa talking among themselves while in their separate space pods, basically going over Raditz’s situation and analyzing all the data the Saiyan sent back to them through his Scouter about the Z Fighters. One key piece of information Vegeta and Nappa acquired was the existence of a half-Saiyan, half-human hybrid–a child who readers know as Goku’s son, Gohan. As far as Raditz’s data informed them, the offspring of a Saiyan and a human has the potential to be vastly more powerful than a full-blood Saiyan, and when Nappa and Vegeta are discussing how to handle this anomaly, Nappa refers to Gohan as a “Super Saiyan” since his genes evidently make him superior.
Gohan's Humanity is What Makes Him a Super Saiyan
Before the transformation of Super Saiyan as fans know it as today was ever established, the term ‘Super Saiyan’ simply referred to a human/Saiyan hybrid as their power had the potential to exceed a regular Saiyan. While at the time of this chapter’s publication that was as suitable a name as any for this new kind of ultra-powerful Saiyan, it is absolutely insane to think of ‘Super Saiyan’ meaning anything other than what it means today given the transformation’s impact on Dragon Ball and all the subsequent transformations stemming from the initial Super Saiyan form.
Goku effectively redefined Super Saiyan in Dragon Ball lore once he achieved the first transformation while fighting Frieza, and then he went on to unlock even more powerful versions of that form throughout the rest of the series–including Super Saiyan 2, Super Saiyan 3, Super Saiyan God, and Super Saiyan Blue. For that reason, the Super Saiyan transformations have become as iconic as Dragon Ball itself, though the title originally meant something completely different which is how Gohan—being the first half-human, half-Saiyan child born in this universe—technically claims the title as Dragon Ball’s first Super Saiyan over Goku.