“Under the ALI [American Law Institute] definition, a de facto parent is a person who shares (at least) equally in primary childcare responsibilities while residing with a child for reasons other than money.  The de facto parent’s assumption of childcare responsibilities must be either with the agreement of the natural parent or result from a parent’s inability to care for the child” (Mason & Zavac, 2002, p.232).
“A de facto parent is an individual other than a legal parent or a parent by estoppel who, for a significant period of time not less than two years, (i) lived with the child and, (ii) for reasons primarily other than financial compensation, and with the agreement of a legal parent to form a parent-child relationship, or as a result of a complete failure or inability of any legal parent to perform caretaking functions, (A) regularly performed a majority of the caretaking functions for the child, or (B) regularly performed a share of caretaking functions at least as great as that of the parent with whom the child primarily lived” (American Law Institute, 2002, p.107-108).

Mason, M.A., & Zayac, N. (2002). Rethinking stepparent rights: Has the ALI found a better definition? Family Law Quarterly, 36(2), 227-254. American Law Institute. (2002). Principles of the law of family dissolution: analysis and recommendations. Neward, New Jersey: LexisNexis.