Definition. The tightest bid/ask spread, in basis points of mid-price, seen at any single moment within the bucket.

Interpretation. Represents the cheapest instant to cross the spread, i.e. best-case trading cost in the period. Read alongside the average and max to see the full range of spread conditions.

Technical. Minimum of (bestask - bestbid) / mid_price, in basis points, within the resolution bucket.