Owing to the fact that a Living Reviews in Solar Physics article may evolve over time, we recommend to cite the article as follows:
David F. Webb and Timothy A. Howard,
"Coronal Mass Ejections: Observations",
Living Rev. Solar Phys. 9, (2012), 3. URL (accessed <date>):
The date given as <date> then uniquely identifies the version of the article you are referring to.