What is the definition or description of: thyroid diseases?

Various. You can have problems with they thyroid making too much or too little thyroid hormone. You can also have problems with it being too large (goiter) of having nodules or cysts in it. There are also occasional thyroid cancers. Most thyroid problems can be easily diagnosed and treated by a qualified doctor.
Thyroid Disease. It can be many things: thyroiditis, low or high hormone levels, tumors, etc. See your doctor for answers.