My husband, 57, has a lump the size of a plum on the left of his neck near the adam's apple. It's painless. No other symptom. What can that be?

Get to a doc! I agree with the previous answer. Though this may be something benign (not cancer), you need this evaluated by ultrasound to start, followed by either more imaging or biopsy. Cancer is my main concern; you need to get your husband in to see a doc as soon as possible! i hope it turns out to be something benign, which is certainly possible. Good luck!
Thyroid /lymph node. A plum is good size lesion. Around the adams apple is the thyroid or lymph nodes. He needs a good exam inside the throat and the neck by an ENT specialist and this lesion will likely need a biopsy. Painless does not mean it's not serious.
Needs evaluation. A plum-sized mass in the neck is never normal. Possibilities include lymph node enlarged by infection or malignancy, thyroid nodule, thyroglossal duct cyst, vascular abnormalities, and benign skin cyst, among others. Such a mass requires prompt assessment; may need biopsy, imaging, ultrasound, or other tests; and quite possibly will warrant removal.
Likely cancer. A lump in one side of the neck can mean swelling of the lymph node due to infection or to cancer. A pain less swelling of the lymph node in the neck is more likely cancer than infection. Cancer of the head and neck regions tends to spread to lymph nodes in the neck. I recommend that your husband see a doctor quickly for evaluation and biopsy if neccessary.