suppletive

adj

1. Making up for deficiencies; supplementary; suppletory.
2. Supplying an etymologically unrelated word with forms, or which is used as one of its forms, by suppletion (for example, in English, better and best are suppletive forms of good).

   “... of optimus, meaning “best,” which in turn is the superlative suppletive form of bonus, “good.””

3. Relating to laws which can apply in a contract if the parties choose to agree to this.

   “...I deem little short of frivolous the contention that the Equal Protection Clause prohibits enforcement of marital obligations, in either the mandatory or the suppletive form.”

noun

1. A form used by way of suppletion.