Contract compliance means adherence to contractual agreements. In procurement the term has two meanings, depending on who is meeting the contract.
Buy side. The organisation actually buys through existing contracts, on the agreed terms. The metric is the share of spend under contract. The counterpart is off-contract spend.
Supplier side. The supplier delivers the promised service on the agreed terms, measured for example against service levels. This topic belongs to supplier performance management.
Note on usage. Without context it stays open which side is meant. In German the term "Vertragstreue" is sometimes used. The term describes a management metric, not a legal assessment.
Data basis. Both sides need contracts in structured form and spend or performance data that can be reconciled against them.