The term open source means you have to, at least, have the right to use, modify, and redistribute modifications. It's more than just being able to see the source.
No, it doesn't. No matter what certain closed-source companies might want you to believe.
There is a difference between Open Source and Free Software, but it's ideological, not technical (as pertaining to the rights granted to the users of the software).
Unfortunately the term "open source" as used by many people is not necessarily synonymous with the official OSI definition available here.
The OSI definition is for most intents and purposes equivalent/compatible/similar to the FSF definition of "free (as in freedom) software" available here https://www.gnu.org/philosophy/free-sw.html
For an official example of the use of the term "open source" to mean "you can read the source but are not allowed to do anything else with it" see the 2007 announcements about QNX. OSI explicitly declared there is no (OSI) open sourcing in what QNX did at the time. https://opensource.org/node/195
For an example of people using the term to mean only "you can read the source" look no further than the discussions related to the recent leaking of Amiga 3.1 source code where people were debating if an effort can be made to convince the copyright owners to "open source" the code:
"Correct me if i'm wrong, but open source doesn't have to equal FOSS. I'd be inclined to agree with you if it were FOSS, but just being open source could provide 'assistance' to Cloanto who can incorporate peoples ideas/fixes but still retain the right to sell it when others don't. Managed well, could it actually be beneficial to Cloanto and the broader Amiga community?"
"You are correct. FOSS and Open Source are not the same. Permissive licensing doesn't have to be attached to the opened source."
"This is true, but nobody suggested it should be FOSS, me included.
Open sourcing of any kind just isn't going to happen without some sort of miracle."
Unfortunately the term "free software" is a terribly ambiguous one (and Richard Stallman and the FSF are to blame here since they knew about the homonimity of the word "free" beforehand)
And even more unfortunately "open source" has for a long time become ambiguous. You can not rely on "open source" meaning "OSI open source" to everyone you talk to.
And members of both organisations and their supporters spend a lot of time fighting about the meaning of each others terms.
So we are stuck with the horrible term "FLOSS", an abbreviation of "Free & Libre & Open Source Software" to roughly define what we actually want to talk about: both copyleft ( https://www.gnu.org/copyleft/ ) and copyfree/permissive ( http://copyfree.org/standard ) licensed software (and in some cases also public domain).
QNX is just another company hijacking the term. I don't know which Amiga thread you're referring to, but the people who were 'debating if an effort can be made to convince the copyright owners to "open source" the code' had it right.
You can complain about ambiguous words and RMS having chosen "free" all you want, but these are the terms we have, and we shouldn't give them up too easily.
