A true relapse may be differentiated by the occurrence of new symptoms directly due to new lesions in the brain or spinal cord.

An exacerbation may be considered as a transient reactivation of inflammation in old lesions, which does not indicate a progression of new disease

I always thought they were one in the same

Apples and oranges, missj. Lesions and Disease progression have little to do with one another. When you have an exacerbation, it means you have inflamation. A new symptom may arise and your old symptoms could also raise their ugly head.

There really is no method to it's madness.
If they could figure it all out, we'd have a cure by now.

I always thought they were the same, too. missj.

Things I've read always consider them to be synonyms.

~ Faith

I've never understood these terms at all, no matter how many times the veterans on these forums try to explain it to me...

Another one that confuses people is "MS is always active."

True: It never goes dormant. It can be stable, very slowly progressing, moderately progressing, or quickly progressing. It may not show up in relapse for years, even after your MS dx.

Then it can cause symptoms from heck, then it can go slowly again. You recover, and years later a problem again.

No rhythm or reason behind it. It does what it very well pleases. We can get pseudo-relapses too. They are not considered true relapses.

Example would be being in heat or hot sun and getting your symptoms to feel worse, like a true relapse is coming on. But cool off and don't overdo it, and it stops those problems in a few hours. Sort of like a fake relapse.

Relapse, exacerbation, and flare, are used as the same word and are the same. Those three have the same meaning.

During a relapse, a new symptom, or new lesion may appear somewhere (brain or spine). Also some old lesion/s may get inflamed (or attacked) again.

If the lesion/s get attacked too often and don't remyelinate even a little, they turn into black holes visible on MRI of the brain. A dead part (black hole).

It does get confusing. For years I mixed them up.