Primus is the planet Cybertron.
Primus may well be the first and most powerful Transformer ever (alongside Unicron) being referred to by most Transformers as the Protector, The Progenitor, or the God of Light, as opposed to Unicron who is often called The Chaos Bringer, the God of Darkness or the Destroyer of Cybertron.
Primus rarely enters his robot/Humanoid form, when he does it is often a case of great Galactic turmoil and unrest. One such example was when a rapidly expanding black hole or vortex threatened to not only destroy Cybertron but the entire galaxy, however Primus was able to stop it.
Primus himself has been shown as an Orb of Light (in Transformers : Energon) and his true body is often not shown - if this is because usually the individual talking to him is standing inside his body is a matter one can only specuate upon. But this means a communion with Primus is often conducted via a proxy of some kind such as the Minicon OverRun, The Matrix of Leadership, or the super computer Vector Sigma.
Rodimus Prime's body type is supposed to be directly related to Primus' original appearance before he was trapped inside the Planet Cyberton.
When a glowing orb is seen that represents Primus or some kind of Energy chamber that is shown to be the heart of Cybertron this is sometime referred to as Primus' Spark. This leads to all sorts of rampant speculation therefore what the relationship should be taken to be between any similar being or device such as The Allspark, The Matrix (Sometimes called "The Greater Matrix" or jokingly referred to as Robot Heaven), The Matrix Flame, The Matrix of Leadship (and Creation), Vector Sigma, The Plasma Energy Chamber and The Oracle.
So perhaps the simplest way is to do what Bumblebee suggests and simply call him the Progenitor.