Atheist is purely a lack of belief in a god.
Buddhists are atheists, but believe that there is a higher consciousness that connects us and everything else to the world and the universe.
So in that sense, buddhists are spiritual atheists.
The difficulty arises because there is no clear definition of spirit or spiritual. Whenever I ask someone who tells me they are spiritual they usually include the words spirit or spiritual....
Now don't get me wrong, I like spirits. A nice single malt or a JD and after enough I may even see spirits, but I haven't got a clue what they are.
I think we have to keep in mind that when it comes to words (and our labels are words) that there is always the denotative and the connotative meanings, and they don't always match. How the dictionary defines certain words (like spirituality) is rarely how the average English user defines it... particularly when it comes to etymologies and common vernacular.
To some, spiritual just means "connected" to something bigger than themselves. That could be our connection to the earth via oxygen, sunshine, food and water...
So denotative(lyl) (at least for now), yes it's a contradiction. But connotative(ly) it's all part of the growing experience we are having with who we are and how we fit into the scheme of things based on our new knowledge and understanding of science.
Carl Sagan used to say he had a spiritual feeling looking up at the Cosmos. His credentials were impeccable. If you disconnect the word spiritual from supernatural and use it mean a sense of wonder about the universe and our world, then there's no problem. If you say you are an atheist but run around with phoney spiritual gurus or whatever, then you do have a problem. Personally, I don't like the word and don't use it because of its connection to religion and new age mumbo jumbo. But that's me.
It's like this... people can believe in the power of prayer - humans perhaps have abilities to send "positive vibes" to people, and perhaps aid in healing the sick. (Mind over matter, that sort of thing.) That does NOT mean that there is some kind of all mystical power involved.
I was intrigued so I had to do some research.
The best and most concise definition I found was on their website, The Center for Spiritual Atheism.
Generally, Spiritual Atheists are people who do not believe in a literal "God" (thus the term "Atheist" ), but still consider themselves to be (often deeply) "Spiritual" people. - site
"Spiritual" has differing, and perhaps conflicting, definitions. To many, it is intrinsicly linked to religious outlook, even if not adherence to an organized religious organization. But by my favorite definition, it absolutely does not contradict atheism. It has nothing to say one way or another about atheism, but they certainly can be compatible . That definition is: "Spirituality = an individual person's values and their priorities." When those two are largely in sinc with each other, one is living a spiritually healthy life. Values are what one believes to be important and good. Priorities are what you invest your attention, time, and energy into ahead of other things.
I get testy when people claim values depend on or stem from religion. Bullshit. Things like altruism, respect for others, integrity, forgiveness of self and others for mistakes, short-sightedness, inconsiderateness, etc. Are all values well worth having and thinking about, and religion lies by claiming it invented them.
It means you are as confused about labels as an a/gnostic a/theist".
It means you want to have your spiritual cake and deny eating it too.
It means you want all the benefits of religion without the drag of admitting there is a god.
It means you don't like one word when two will do.