Recurrence and relapse
The cancer has come back after a period in which it could not be detected. 'Recurrence' is the usual word for solid tumours and 'relapse' for blood cancers; they mean the same thing.
Recurrence can be local (at the original site), regional (in nearby lymph nodes) or distant (metastatic), and the interval before it is informative: early recurrence suggests aggressive, resistant disease, while late recurrence in breast cancer or melanoma reflects cells that lay dormant for years. It happens because microscopic disease survived the original treatment; adjuvant therapy exists to reduce this risk, and ctDNA tests after surgery can now detect the residual disease months before a scan would. Recurrent disease is treated according to where and when it returns, ranging from a second attempt at cure for an isolated local recurrence to systemic therapy for metastatic recurrence.