In classification of vector fields, one of the 4 different type vector fields is "solenoidal and irrotational vector field" (both divergence-free and curl-free).
If solenoidal and rotational vector fields are same thing, then it means the vector field is "rotational and irrotational vector field" at the same time. But we just want to indicate it is divergence and curl free.
Is it the reason why synonym names(solenoidal and rotational) are used, to avoid confusion, or do they have different meanings?
Solenoidal vector field and Rotational vector field are not the same thing. A Solenoidal vector field is known as an incompressible vector field of which divergence is zero. Hence, a solenoidal vector field is called a divergence-free vector field. On the other hand, an Irrotational vector field implies that the value of Curl at any point of the vector field is zero. i.e, the vector field is conservative.