Strictly non medical definitions.
Weakness is due to lack of muscle strength. Cause of that can be lack of use (atrophy) or disease (like myositis but there are other diseases that weaken muscles).
Fatigue and tired are synonyms but I differentiate them.
Fatigue is chronic condition that can be caused by many different things: disease, lack of sleep, etc.
Tired is usually from overexertion. Get a good nights sleep and the next day you feel OK. If you are tired all the time it is fatigue.
Anyway, that is how I think of the terms.
Mixed connective tissue disease (systemic lupus, scleroderma, polymyositis), Raynauds phenomena, Hypertension, Barrett's esophagus.
Meds: prednisone (7mg & tapering), 100mg imuran, lisinopril 40mg, maxide 37.5/25mg, norvasc 5 mg, folic acid, potassium, aciphex 20mg, multi vitamins.
Maintain your optimism and you can beat the odds.
"Although the world is full of suffering, it is also full of the overcoming of it." Helen Keller