手边正好是IoA的书，glossary里有lapse的解释，但没有lapse rate的，仅供楼主参考吧：

A life insurance contract lapses if the policyholder terminates a contract early due to non-payment of premiums without the company making a surrender value payment to the policyholder. Some companies also use "lapse" to describe policies that have been surrendered.

另，这是英国的说法，要看楼主怎么用