Let's first focus on the last word (bill) and then work our way to the 'beginning'...

It all started with an idea that a change needed to happen. When 'ideas' are introduced into the realm of a state's legislative body, ideas often become Bills. The definition of a Bill is 'a draft of a law presented to a legislature for enactment'. Sometimes, an originating bill is also known as the law that evolves from it (as in the 'G.I. Bill'). If all goes well, a Bill becomes an Act (the formal product of a legislative body), then the Act becomes a Statute (a law enacted by the legislative branch of a government).

And now, to the 'MISSing Angels' part of it:
On July 27th, 1994, Joanne Cacciatore's precious daughter Cheyenne was born still. Joanne received no birth certificate from Arizona acknowledging her 'motherhood,’ nor the birth of Cheyenne, and felt this to be wrong. She wrote her legislator and asked that a change be made - and thankfully - she didn't stop there. Since Cheyenne's death, Joanne founded a group for parents who lose infants called M.I.S.S. (Mothers In Sympathy and Support). Joanne dedicated more than a year of her life in Arizona's Senate and House chambers meeting with legislators, public health officials, and organizing a task force in 1999 with volunteer members including Paula Mikkelson, Heidi and Carl Brashears, Kipp and Susie Charlton, Guillermo Gutierrez, M.D., David Garard, Sgt. Randy Force, Dr. Mark Fischione, and Cindi Nannetti. M.I.S.S. Foundation's families were the initiators and supporters of this legislation.

A portion of the text in the original legislation reads:
Section 36-329.01, Arizona Revised Statutes, as added by this act, may be cited as the "Missing Angels Act".

The actual name of the legislation was meant to be 'MISSing Angels' in order to honor the M.I.S.S. Foundation families and Cheyenne Cacciatore for whom the organization was originally founded. Due to an administrative error, however, the legislation was formally documented in an incorrect manner.