When an offer has been accepted by the seller but no legal contracts have been exchanged.
When the vendor has accepted an offer for his home but contracts have not yet been exchanged. Either party may still withdraw from the transaction. The agent will often display a board saying "Sold subject to contract" at this point
A property is under offer when a homebuyer has made a formal offer to purchase the property but the homeowner has not confirmed whether to accept. If the offer is rejected then the property remains on the market until the next formal offer is made at which point the homeowner must again decide whether to accept the offer or reject it. If the offer is accepted then it becomes 'Sold Subject to Contract' (Sold STC).