Accumulation is a wyckoff-method term for the sideways period that follows a downtrend, where large buyers absorb the supply still coming from sellers. Price goes nowhere while ownership changes hands.
The chart clues usually cited are declining volatility, volume rising on up bars relative to down bars, failed attempts to break lower, and eventually a wyckoff-spring that shakes out the last sellers before the range resolves upward.
The concept is reasonable but the labelling is almost always retrospective. A range that later breaks up gets called accumulation; the identical range that breaks down gets called distribution. Unless your rules define accumulation in advance and in testable terms, you are describing the past, not forecasting.
Related: distribution, wyckoff-accumulation, wyckoff-method, trading-range, consolidation