High-water mark
The highest balance or equity an account has reached; trailing drawdowns are measured from it.
If your account peaks at $104,000 and falls to $101,000, the high-water mark stays $104,000 until a new peak is made. Under a $5,000 trailing drawdown, the floor is the high-water mark minus $5,000, here $99,000.
Whether the mark updates at each day’s close, after each closed trade, or with live equity is the main difference between trailing drawdown types.
Related
General definition. Rules differ between firms and change over time; your firm’s current terms always take precedence.