Among the bewildering number of Buddhist traditions and lineages, and even more personal opinions, the question always stimulates interesting discussion.
HH Dalai Lama and other teachers answer the question by defining three axioms of Buddhist philosophy, known as Three Seals.
Below is the shortest and simplest version of the Three Seals I have found.
"...the three Dharma seals, or to put it in plain English, three basic facts about Being. They spoke to me … because of their universality, which extends far beyond the boundaries of religion.”
(From BUDDHA by Deepak Chopra pg 269)
1. DUKKHA (suffering)
Life is unsatisfactory. Pleasure in the physical world is transient. Pain inevitably follows. Therefore, nothing we experience can be deeply satisfying. There is no resting place in change.
2. ANICCA (impermanence)
Nothing is permanent. All experience is swept away in flux. Cause and effect is endless and confusing. Therefore one can never find clarity or permanence.
3. ANATTA (no-self)
The saparate salf is unreliable and ultimately unreal. We apply words like soul and personality to something that is fleeting and ghostly. Our attempts to make the self real never end but also never succeed. Therefore, we cling for reassurance to an illusion."
Other teachers and laity refrain from defining themselves as Buddhists all together.
Whas Shakiamuni Buddha a Buddhist?
There are also many who, while being loyal to their own religious tradition, also subscribe to and practice Buddhist tenets.
Theravada Buddhism, the oldest and (arguably) least sectarian, seem to be more a system of philosophy and psychology than religion (does not concern itself with supernatural), and easily "fits" with various religious traditions.
With metta
Gotami
