Simplified forms of a language that use limited vocabulary and grammar are referred to as:

The term that refers to simplified forms of a language using limited vocabulary and grammar is Pidgin. Pidgin languages typically develop in situations where speakers of different native languages need to communicate with each other, often for trade or other practical purposes. In these contexts, a new form of communication emerges, which combines elements from the various languages involved but simplifies their structure and vocabulary to make it more accessible to all speakers.

Pidgins are often characterized by their function as a lingua franca, which means they serve as a common language among people who do not share a native language. Over time, if a pidgin language gains a stable, generational use among a community, it can evolve into a creole— the next stage where it becomes a fully developed language.

In contrast, other language forms mentioned in the question have different characteristics. Creole languages are more complex and stable than pidgins, developing fully-fledged grammatical and syntactical structures after prolonged use in a community. Slang refers to informal language and specific vocabulary among particular groups but does not imply a simplification of grammar. Patois generally refers to regional dialects, especially those that deviate significantly from the standard language. Finally, dialects refer to variations of a language that include