(noun) 1. a set of beliefs concerning the cause, nature, and purpose of the universe, especially when considered as the creation of a superhuman agency or agencies, usually involving devotional and ritual observances, and often containing a moral code governing the conduct of human affairs. 2. a specific fundamental set of beliefs and practices generally agreed upon by a number of persons or sects: the Christian religion; the Buddhist religion.
There are more definitions to be sure, but these are more than adequate to ask the question, "Is the practice of Islam by ISIS a religious practice?
To begin answering this, the first part of the definition of religion must be considered: do ISIS terrorists hold beliefs about the cause, nature and purpose of the universe?
Human nature is called such because there is a universal component as to how we behave or are motivated. It's part of what makes us human.
We now can ask: how would human nature explain what motivates a suicide bomber to kill him- or herself in such an utterly violent manner in order to kill strangers?
Human nature, in general, is instinctual when it comes to survival, thus it's known as the 'survival instinct.' Most people would not end their lives if they believed that there was nothing coming for them after the suicide (i.e., eternal life in Paradise). The notion that there is eternal life after death is, admittedly, a religious notion, most would agree.
So committing suicide goes against our instincts, but if religion justifies it [for purposes of jihad] then it isn't viewed by the suicide bomber as killing the self, it's viewed as a way of ensuring eternal bliss (if having 72 sex partners can be considered bliss rather than work).
Islamic scripture goes into elaborate discussions about jihad. This all came from Mohammad, the so-called prophet, who was a desert terrorist and pedophile. He robbed desert caravans, killed non-believers, especially by beheading them, and taking the wives of the dead as sex slaves.
Does this sound anything like ISIS?
Islam defines the nature of the universe as the work of Allah, the pagan moon god, originally, then stolen by Mohammad like he stole ideas from the Jews. So the next part of the definition seems to confirm that Islam is a religion.
ISIS practices Islam stricter than moderate Muslims practice it by far. They actually believe that detonating their suicide vests doesn't kill them in the same way we may envision death, but sends them on an eternal trip to Paradise.
And it isn't as if it is merely a handful of ISIS terrorists who believe in jihad tactics, including suicide bombing, it's millions of Muslims across the world, and all of the ISIS, al Qaeda, Hezbollah, Hamas, Taliban, Al Shabaab, Al Nusra, etc. etc. who believe in jihad, including suicide bombing.
The main difference, it seems, between Islamists and moderate Muslims, is that the former is certain in their religious convictions, the latter is still on the fence.
That folks, is a religion. Ands ISIS is, indeed, Islamic to the nines.