Ind. Code § 32-29-10-4

Sec. 4. As used in this chapter, "supplemental indenture" means an instrument or indenture executed to supplement the original security instrument, defining one (1) or more series of bonds, notes, or debentures secured, or to be secured, by the series mortgage, specifying property subject to the lien of the series mortgage or in another manner supplementing or amending the original security instrument.