Break-even revenue is the sales level at which profit is exactly zero — the minimum needed to recover all fixed costs. Above it you are profitable, below it you lose money. It is a management-accounting measure, not a tax concept.
Break-even is a management-accounting measure; there is no governing tax statute.
Costs that move with sales (purchases, materials, sales commissions) are variable; costs incurred regardless of sales (rent, permanent salaries, depreciation) are fixed.
As a rule of thumb, 80% or below leaves room; above 90% a small sales dip tips you into loss. The benchmark varies by industry and cost structure.