Pending usuall means that an offer has been recieved, but the deal has not closed yet. With a short sale it may be that the owners have accepted the offer but are waiting for their lender to approve the contract. Then again everyone may have accepted the offer and they are just waiting to close.
Short sale pending means that the sellers have accepted the offer and it has been sent to the lender/bank, but they have not approved it yet. Often it takes quite a while for them to do broker price opinions and appraisals of the property to agree or disagree with the price. Then they may accept or counter the offer that was signed by the sellers.
On RMLS say is "SSP" mean Short Sale Pending, the seller has accepted an offer and send the offer to seller's lender or lien holder to get approval, waiting to hear from the seller 's lender or lien holder to approval the offer or not. can not be close until all parties agree (buyer, seller, and seller's lender or/and lien holder)
Short Sale Pending (SSP) indicates the seller has accepted an offer, but the 3rd Party Lender has not yet provided approval of this offer. The transaction will not close/record until the 3rd Party approves the deal.