If one parent unilaterally moves out of state with the child without the permission of the other parent and without an Order of the Court, then you must act immediately to force the return of the child. If the child resides out of state for more than six (6) months then under the laws of most states (most states having enacted the Uniform Child Custody Jurisdiction and Enforcement Act, or some variation of it), that other state will now be the "home state" and have jurisdiction over the child. You should consult with an attorney about the appropriate action to bring before a Court in order to obtain an order requiring the child to be returned (which will then have to be brought to the other state and enforced, through the use of law enforcement if necessary).
If both parents do not agree to allowing one parent to move with the child to a different state, then the parent that wants to move is required to file a Petition for Removal. If the Court has already ruled on the custody of the minor child, in a Paternity or Divorce judgment, then this it typically done with the filing of a Complaint for Modification.
The standard for approval of a request for Removal is actually a two-part test as defined in the landmark case Yannas v. Fronditsou-Yannas, 395 Mass. 704 (1985). The Court in Yannas created a two-part test:
First, the parent requesting removal must first show the court that there is a "good, sincere reason" for the move. This is often described as the "real advantage test." Examples of acceptable "good, sincere reasons" are a lucrative job opportunity, significantly greater family support, or a new spouse who resides in another state. In Pizzino v. Miller, 67 Mass. App. Ct. 865 (2006) the Court found that "a sincere desire to be with a spouse is, per se, a good and sufficient reason".
The second part of the test requires the Court to evaluate whether the move is in the best interest of the minor children. Typically if the first part of the test is meant, the "real advantage" likely carries over and demonstrates that the move is also in the best interest of the minor child. Meeting the second part of the test may be more difficult, however, in the case where both parents are significantly involved in the minor child's life, such as when there is a joint physical custody arrangement.