To answer this question, we first have to define the various types of forks in Bitcoin.
Software Fork - Just what it sounds like, it is a copy of source code from a software package which developed independently from the original creating a distinct piece of software.
Normally when people are referring to a fork in Bitcoin they are referring to a Soft Fork or a Hard Fork which is different from a Software Fork.
Soft Fork – A change to the Bitcoin protocol where previous blocks are made invalid. The soft fork is backward-compatible since nodes running the old software version recognize the new blocks as valid. The blockchain is not split in this scenario. - http://archive.is/pTDkE
Hard Fork – Usually the result of an upgrade to existing software or a software fork, which causes new blocks in the existing chain to be incompatible with the old blocks without running the updated software. This splits or forks the blockchain into two separate chains. One maintained by the outdated software, and one maintained by the upgraded software.
When speaking to many Core supporters, they inaccurately reference "software forks" in order to define Bitcoin Cash as an altcoin. They will say that an alternate version of the Bitcoin software was created so Bitcoin Cash is an altcoin. This is simply not true.
An altcoin is any blockchain that is created that does not contain the original (genesis) block mined in January 2009. It is a software fork that does not include a hard fork of the original Bitcoin blockchain. Again, Bitcoin Cash is a hardfork.
According to investopedia a hardfork…
essentially creates a fork in the blockchain, one path which follows the new, upgraded blockchain, and one path which continues along the old path. Generally, after a short period of time, those on the old chain will realize that their version of the blockchain is outdated or irrelevant and quickly upgrade to the latest version.https://www.investopedia.com/terms/h/hard-fork.asp
So next time someone from core tries to tell you Bitcoin Cash is an altcoin, set them straight or refer them to this steemit post. Per definition, Bitcoin Cash is NOT an altcoin and instead an upgraded version of the Bitcoin blockchain. We just chose to change names to avoid confusion, knowing there would be two separate chains.