non-pitting edema
non-pitting oedema

Non-pitting edema is observed when the indentation does not persist.

Non-pitting edema does not leave this type of dent when pressing on the swollen area.

The "myxedema" part of the name simply refers to the non-pitting edema common to hypothyroidism.

The two signs that are truly 'diagnostic' of Graves' disease (i.e., not seen in other hyperthyroid conditions) are exophthalmos and non-pitting edema (pretibial myxedema).