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A boson has a symmetric complete multiparticle state under exchange of identical particles. The symmetry includes internal degrees of freedom as well as position. Under the locality and positivity hypotheses of relativistic quantum field theory in three spatial dimensions, the Spin-statistics theorem relates this exchange behavior to integer spin.
Symmetry permits several bosons to occupy one single-particle mode. Creation into an already occupied mode has the bosonic square-root occupation factor, contributing to stimulated processes and Bose-gas behavior. Photons and helium-4 atoms are examples at different descriptive levels. Symmetry does not force every preparation to exhibit a particular amount of bunching or to form a condensate: measured correlations also depend on the state, interactions and detector resolution.
The term "boson" refers to a category of subatomic particles that obey Bose-Einstein statistics, which means they can occupy the same quantum state as other bosons. This characteristic distinguishes them from fermions, which follow the Pauli exclusion principle and cannot occupy the same state. Bosons include force carrier particles and have integer values of spin (0, 1, 2, etc.).