The span an entity reports on — which it often chooses, within limits.
An accounting period is the span an entity draws its accounts for. Unlike the tax year, it is frequently the entity's own choice within statutory limits, and companies pick year-ends for commercial reasons: after a seasonal peak, or aligned with a parent abroad.
The consequence for anything computing deadlines is that a jurisdiction-level answer may not be enough. Where a filing deadline is expressed relative to the period end, two companies in the same country have different deadlines and no calendar keyed to the country can produce either.
First and final periods are also frequently not twelve months, which breaks the assumption that a year end recurs annually on the same date.
The tax year is fixed for the jurisdiction. The accounting period belongs to the entity and may not align with it.
See also tax year · filing deadline · deadline
We hold this as not held — an entity-level fact, outside a jurisdiction corpus. See what we hold.