it is C with an extensive library.
BUT because there are soooo many libraries and a majority of people ONLY use the subroutines that were prewritten for them in the libraries, it is in effect sort of like BASIC. I bet you many of the people using the Arduino think that Arduino is a language and is similar to BASIC.
That's very subjective. People thinking it's similar to BASIC doesn't make it sort of like Basic.
Maybe it's my background in computer science, but Arduino and C have the same syntax, same type system, same execution semantic and share most of their core library (avr-libc).
BASIC as a completely different syntax, a completely different type system, the execution semantic is remotely similar, and the core library is not even comparable (what version of basic are we talking about anyway?).
I know, I sound a bit pedantic
but saying that it's similar to basic is somehow misleading.
And don't get me wrong, I LOVE the Arduino. I own a board and I use the *libraries* quite often.