To receive another gladly into a place that is yours. Welcome is a decision the host makes before the guest arrives.
Old English wilcuma, joining wil- (pleasure, desire) with cuma (a comer, from cuman, to come), named the decision to receive another gladly, made before the guest arrives.
The receiving of another gladly into a place that belongs to the host, a decision made before the guest has even arrived to need it.