The trigger point at which a tranche becomes exposed to losses-due-to-default in the underlying portfolio. For example, a 3-7% tranche has an attachment point of 3%. Until the accumulated losses-due-to-default of the underlying portfolio reach 3% of the total initial pool, the tranche will not be affected. However, any further loss (beyond 3%) will be deducted from the tranche’s notional, until losses reach the 7% detachment point.