In Matter of Alison D. v. Virginia M., 77 NY2d 651 [1991] the Court of Appeals denied visitation rights to the former partner of a same-sex relationship on the grounds that, as a biological stranger to the child, she could not be deemed a "parent" under Domestic Relations Law 70. The petitioner had argued that, although she was not a "parent" either biologically or by virtue of adoption, her substantial relationship to the child resulted in her being a "de facto" parent or that she should be viewed as a parent "by estoppel." The court rejected the application of equitable estoppel and concluded that, where the biological parent is fit, the state will not interfere with that parent's custodial choices. (See Matter of Ronald FF., supra; Matter of Bennett v. Jeffreys, 40 NY2d 543 [1976].) In her dissent, Chief Judge Kaye noted that the word "parent" is not defined by statute. Where a term is undefined, it is for the courts to give definition to the term to effectuate the legislative purpose. She concluded that the narrow reading of the term "parent," given by the majority, precluded the court from advancing the legislative intent. "The Legislature has made plain an objective in section 70to promote 'the best interest of the child' and the child's 'welfare and happiness.' Those words should not be ignored by us in defining standing for visitation purposes . . ." (77 NY2d at 659 .) The Court of Appeals held that same sex partners who are "biological strangers" to the children are not "parents" for purposes of New York's Domestic Relations Law, and, thus, have no standing under Domestic Relations Law 70 to petition for visitation when the children at issue are properly within the custody of a fit legal or biological parent. In reaching this conclusion, the Court reaffirmed the traditional New York rule that visitation is a modified form of custody, and that, as between a parent and a third person, parental custody of a child may not be displaced absent grievous cause or necessity. (see also, Matter of Ronald FF. v. Cindy GG., 70 NY2d 141, 144). The Court of Appeals also affirmed the Appellate Division's ruling in Matter of Alison D. v. Virginia M. (155 AD2d 11, 14-15) that New York's exceptional or extraordinary circumstances doctrine does not apply to such circumstances.