What is the definition or description of: metabolic acidosis?

Decreased pH: Metabolic acidosis occurs either from primary acid retention, renal (kidney) dysfunction/failure or bicarbonate (sodium bicarbonate) losses. Primary acid retentions cause anion-gap metabolic acidosis from these etiologies (methanol, uremia, diabetic ketoacidosis, para-aldehyde, isoniazid, lactic acidosis, ethylene glycol, salicylates). Kidney's dysfunction leads to acidosis. Finally any loss of bicarb (diarrhea etc).

Increased blood acid: The blood is in balance between acid and non-acid (alkali). Both of acid and alkali levels could change under the influence of the kidneys which we call a metabolic change or by effect of lungs which we call respiratory change. Any increase of acid load exceeding the ability of the kidneys to neutralize as in aspirin/alcohol/antifreeze poisoning, sepsis cause met acidosis, as can kidney failure.