Cancer is basically cells that are growing out of control. Clusters of cancer cells are called tumors. Tumors as small as a few millimeters can be detected by some scans (see Diagnostics for the different types of scans used).
Metastasis is when cancer cells break off from the main tumor and travel to another part of the body and start growing there. Patients sometimes refer to metastasis as "mets".
Once cancer is detected and treatment starts, the patient is monitored to see how effective the treatment is.
When tumors grow, that is called progression. When progression occurs at some point during that may indicate the cancer has changed. There are several variants of ALK+ lung cancer. Genomic testing of either a tumor biopsy (preferred because its more accurate) or blood biopsy (less invasive but also may not be able to detect the cancer) can determine if you have a specific variant.
When tumors remain the same size, that is called "stable". This status is considered a good state to be in since the cancer is not growing. This can indicate the treatment is effective.
When tumors shrink to a small enough level, some people refer to this as NED (No Evidence of Disease). For ALK+ lung cancer, remission is not a term that is normally used.