Oxford Dictionary definition: ‘Marks corresponding to those left on Christ’s body by Crucifixion, said to have been impressed by divine favour on bodies of St Francis of Assisi and others.

Specific case of Louise Lateau:

Louise Lateau was a young girl who suffered from both stigmata and ecstasies. Her first stigmata occurred in April 1868 when she began bleeding from the left side of her chest. The bleeding occurred again the following Friday, however this time she also had wounds on her feet. On the third Friday wounds appeared on her hands. On September 25th small cuts formed on her head forming a bloody crown. These wounds continued to appear weekly for months and years. These cases all signify/represent the marks left on Jesus. For example, the spear through his side, the nails through his hands and feet during his Crucifixion and the crown of thorns placed on his head.