What makes an operating system “Unix-Like”?

I frequently come across the term "unix-like" on the internet on many sites.

There is no standard; it is just in the way it behaves.

But if I were to develop a kernel from scratch, what would make it considered "unix-like"?

Basically, what are the things that make written code like Unix?

 

 

 

 

 

 

출처 : http://unix.stackexchange.com/questions/87686/what-makes-an-operating-system-unix-like