Nope. Its not the definition of open source. Its the defenition of "source availiable"

Real open source defenition : https://opensource.org/osd

Real Free Software defenition : https://www.gnu.org/philosophy/free-sw.en.html

Have a good read. If you want tl;dr, here is it

Open source is a developement methodology in which software is licensed with the permission for everyone to collaborate and develop it. Anyone has the freedom to modify and contribute to it.

Free software is a matter of freedom. It refers to the 4 essential freedoms a software should have, that is freedom to use, modify, share copies and share modified copies for the community however you want without restriction.

FOSS is software that fits both theese defenitions. While practically almost all free software are open source and almost all open source software are free software, but there are few exeptions