De facto basically means according to the facts. It is to be distinguished from de jure, which, more or less, means pursuant to an order from a court.
These two terms often comes into play when reviewing a custody dispute between a parent and third party (such as a distant relative). If the third party has custody pursuant an order of the court, then the third party is the de jure custodian. On the other hand, if there is no court order but that same third party has had the child for a sufficient amount of time, that third party can argue that she is the de facto custodian over the child, meaning that she has had the child for so long that she has, according to the facts, served as the custodian for the child.
Indiana Code 31-9-2-35.5 defines a de facto custodian as follows:
De facto custodian”, for purposes of IC 31-14-13, IC 31-17-2, and IC 31-34-4, means a person who has been the primary caregiver for, and financial support of, a child who has resided with the person for at least:
(1) six (6) months if the child is less than three (3) years of age; or
(2) one (1) year if the child is at least three (3) years of age.
Any period after a child custody proceeding has been commenced may not be included in determining whether the child has resided with the person for the required minimum period. The term does not include a person providing care for a child in a foster family home.