Alternative - most commonly used when looking for more options.
Example: The drink was too sweet, he looked for an alternative (different) beverage.
With regard to the medical field,
Alternative Medicine - any types of healing or treating an illness or disease that is not a traditional medical practice.
Example: A patient has a stomach ache. The doctor gave the patient a cup of black tea. The tea is an alternative medicine. The doctor could have given him an antacid.
Alternating Current - an alternating current (AC) is an electrical current. More specifically, a current that periodically flows in opposite directions.
If you are researching Alternative Medicine, the alternating current may be in reference to a practice called "Electrotherapy".
Electrotherapy is a medical treatment that uses electrical currents to treat / help a wide range of illnesses or symptoms.
I hope this helps you.