Can defecation be voluntary or involuntary?

Defecation is a process by which solid waste material is eliminated from the body through the anus in the form of faeces. 

It can be both voluntary and involuntary. The contraction of the rectal muscles and relaxation of the anal sphincter muscles are involuntary reflexes, but the process of defecation can be voluntarily initiated or postponed by the individual. This is possible due to the conscious control of the external anal sphincter muscle.