Recently I've seen this question asked on the internet, but with no direct answer.
Is Swahili a language?
In order to respond with a good answer, I'll have to consider what exactly is meant with the question: If Swahili wasn't a language, what else could you suppose that it would be?
Swahili is a language, but it also refers to a group of people. The Swahili people are an ethnic group living in the coastal areas in Kenya, Tanzania, Mozambique, and Zanzibar. They're also the first people to speak the Swahili language. Despite Swahili having evolved from earlier Bantu languages, it's not a dialect, and even though it has many loan-words from Arabic, Hindustani, Persian, Portuguese, and Malay, it is a fully-fledged language and not a creole.
Why Swahili Isn't A Creole
Creole languages are hybrids of different languages that have arisen when people with different languages have met throughout history. They exist all over the world and are generally relatively simple compared with the original languages from which it came.
Swahili is a language that belongs to the "Bantu" group of the Niger-Congo family of languages. Before the arrival of foreigners in the coastal regions of Kenya where it was spoken, it was purely a Bantu language with no (or only local) loan-words from other languages.
With the establishment of Arabic trade colonies in East-Africa, Swahili's ancestor began borrowing from Arabic, but also from several other languages of the Middle-East and South Asia.
Today, towards 30% of Swahili vocabulary is said to be of Arabic origin.
This doesn't make Swahili a Bantu-Arabic creole language, however, because it isn't a simplified "compromise" between two languages, but rather one, well-founded Bantu language with loan-words which all have to adapt to Swahili grammar, pronunciation, and syntax.
In other words, Swahili is very much a language in its own right, and not a creole.
And if you aren't persuaded by that argument, think of English. A whopping 60% of English words are loan-words from French and Latin. But you wouldn't call English a "French-Old English creole" would you?
So Is Swahili A Dialect?
There's no clear distinction between when you call a language "dialect" and when it becomes a language. Some, very different languages, such as different forms of Chinese or Arabic are considered dialects even though they're quite dissimilar, whereas mutually intelligible languages such as Czech and Slovak, or Danish and Norwegian are considered languages even though they could easily pass for dialects.
With Swahili, however, there isn't really a lot of confusion.
Swahili belongs to the Bantu group of languages as mentioned before, but Bantu isn't a language in itself, so it can't be a "dialect of Bantu" just like English isn't a "dialect of Germanic".
Swahili is a unique and singular language. It does have dialects on its own, though. Swahili speakers in Tanzania, Kenya, and Uganda all speak differently, but there is a main "official" Swahili language that is considered "correct".
Some assume that Swahili is a dialect of Arabic or something similar.
Well, it isn't. Swahili and Arabic aren't even remotely related!