Sennin and Sage are the same thing. However, I will argue to death that characters are referring to different ability when using Sennin or Sage.
For example, Madara is currently using Sennin powers, however he does not have Sage Mode.
-He does have sage mode,didn't you see those rings on hashiramas face fused inside his body?that's sage mode.
then what is SANNIN also??
Sorry. Bad example. Obito, who was said to have "taken part of my sage mode" by Madara does not have "Sage Mode" but had taken a part of the Sannin power, AKA: Rikudou's power.
Sorry. Bad example. Obito, who was said to have "taken part of my sage mode" by Madara does not have "Sage Mode" but had taken a part of the Sannin power, AKA: Rikudou's power.