www.alphadictionary.com

Strictly speaking, this is an Arabic word transliterated into English. Inside it, you can see the word "jihad," especially if you omit vowels which is often the case in Semitic languages. Historically, the word goes back to the early 18th century and refers generally to one who is on jihad, and specifically to a guerilla fighter. The word became useful for the Afghan guerillas who fought the Russians successfully.

So much is clear. The difficulty arises with contemporary usage which seems to slip all over the place. I've read a good deal about the Middle East in the last few years and "mujadin" in various spellings is obviously used in the above sense, but also seems to refer to any soldier in a militia or small band (always Arabic), but often of various countries and sometimes in a non warlike situation.

"Jihad" itself is debatable as to how much it applies to war. So far as I can tell, its etymology goes back to war against non-muslims, but many moslem leaders insist that jihad is a search for peace and point to the fact that "Islam" means submission. They interpret that as submission to Allah, while others are more likely for the word to imply using whatever means necessary to submit everyone to Islam.

What is your uunderstanding of Mujahedin?

My understanding is the name applies to Muslim guerilla fighters, generally resisting non-Muslim forces, for instance soldiers in Afghanistan or Iran.