To me- there is: probe (aka electrode) and electronic interface (shield?) to arduino. What I call "sensor" is the active part of the probe. There are probes for different parameters, right. Anyway, terminology. 
I was also interested in making a pH meter to be used to Arduino, but what I found here is only pH probe + interface. To be mounted (connected) to the Arduino uno. That is all. No fun at all. And they grey market here. 