What is the definition of atheism?

The definition of atheism is indeed best captured as a lack of belief in the existence of God or gods. This perspective does not assert a belief in any deities, nor does it engage in the spiritual practices typically associated with religious belief systems. Atheism is often characterized by a position of skepticism towards religious claims and doctrines. It focuses on empirical evidence and rational inquiry, leading many who identify as atheists to refrain from accepting the existence of any divine beings based on the current evidence available.

In contrast, other options describe beliefs or practices that do not align with the definition of atheism. Belief in multiple deities pertains to polytheism, which is the opposite of atheism. A belief system centered on spiritual practices indicates a commitment to some form of religion or spirituality, which is inconsistent with atheism's fundamental premise. Likewise, a tradition of civic duty and social involvement does not inherently relate to the existence of deities or belief systems, making this option unrelated to the definition of atheism. Thus, the correct answer reflects the essence of atheism's stance concerning belief in deities.