Question: I'm a beginner in electronics field . In hybrid and pi and T models we have a resistance called r0 or ro . What's the correct name ? Is it "r-zero" or "r-o (15th alphabet letter)". Pls give references . Different people say different stuff !

Thanks ...

Answer: 
It's a subscripted letter O. \$r\_o\$ is the output resistance of the transistor. It represents the fact that the transistor is not an ideal voltage-controlled current source. The collector/drain voltage does have some influence on the current, and that's what \$r\_o\$ models.
In the models you are referring to, ro (as in the letter) is short hand notation for Resistance Output. More specifically this resistance is due to the Early Effect.