There are multiple definitions - click on the translation that matches your context.
relating to the residue of an estate (=the part of a dead person's property and money that is left after any charges, taxes, funeral expenses, and debts etc have been paid)
Mrs Swanston left her residuary estate to Dr Kershaw's Hospice and not to her children.
residual; remaining after the rest of something has gone or finished
In terms of administration time, the overwhelming majority of legacy income comes from residuary gifts.