lucid

adj

1.
(of language) transparently clear; easily understandable
crystal clear limpid luculent pellucid perspicuous
Derived from:
lucidity
2.
having a clear mind
Derived from:
lucidity
3.
capable of thinking and expressing yourself in a clear and consistent manner
coherent logical
4.
transmitting light; able to be seen through with clarity
crystal clear crystalline limpid pellucid transparent