What is the difference between a mass and a goiter in the thyroid gland?

Goiter is universal enlargement of the ENTIRE thyroid gland, but a thyroid mass is a loculation of tissue in the thyroid gland that could be: fluid/fluid+cellular component/or cellular component only; also, thyroid mass could be benign versus malignant ( regarding to the pathology result ) .

HAHAHAHAHAHAHAHAHAHAHAHAHAHHAHAHAHAA. sorry had to laugh. this reminds me of an inside joke between me and one of my besties. good luck :)

I believe a mass is a solid tumor of sorts,

where as a goiter is an enlargement of the thyroid gland in the neck area.

a "mass" is a collection of cells that are believed to have become cancerous (growing and multiplying beyond control or reason).
A goiter is an enlargement of the thyroid gland caused by abnormal thyroid hormone secretion (usually caused by lack of iodine in your system). It is not a growth/mass of abdormal cells.

Your thyroid gland is one of the endocrine glands, which make hormones to regulate physiological functions in your body. The thyroid gland manufactures thyroid hormone, which regulates the rate at which your body carries on its necessary functions. Other endocrine glands are the pancreas, the pituitary, the adrenal glands, the parathyroid glands, the testes, and the ovaries. An enlarged thyroid gland is often called a "goiter.

The thyroid gland is located in the middle of the lower neck, below the larynx (voice box) and just above your clavicles (collarbones). It is shaped like a "bow tie," having two halves (lobes): a right lobe and a left lobe joined by an "isthmus.". You can't always feel a normal thyroid gland
Patients may develop "lumps" or "masses" in their thyroid glands. They may appear gradually or very rapidly. Patients who had radiation therapy to the head or neck as children for acne, adenoids, or other reasons are more prone to develop thyroid malignancy. A doctor should evaluate all thyroid "lumps" (nodules).

so there really is no difference..

A mass is an encapsulation of usually non-benign cells. A goiter is the abnormal enlargement of the thyroid gland.