When you combine them, `基本 (jīběn)` means “basic” or “fundamental.” Adding `上 (shàng)` creates `基本上 (jīběn shàng)`, which literally means “on the foundation” or “on a fundamental level”—perfectly capturing the English idea of “basically.”
The frequent use of 基本上 reflects a common communication style in Chinese culture that often values nuance and indirectness over absolute declarations. Making a blanket statement can be seen as overly assertive or even arrogant. Using 基本上 is a way to be more precise and manage expectations. If you tell your boss, “The report is done” (`报告做完了`), it implies 100% completion. If a tiny typo is found later, it could seem like you were careless. However, if you say, “The report is basically done” (`报告基本上做完了`), you accurately communicate that the main work is finished, but it's pending a final review. This is seen as responsible and humble. Compared to American/Western culture, where directness is often prized (“Yes, it's done.”), using a qualifier like 基本上 is a standard and often preferred way to convey progress in China. It's not a sign of uncertainty but rather one of thoroughness and politeness, leaving a small margin for unforeseen details without making a promise you can't perfectly keep.
基本上 is an extremely common word used in both formal and informal contexts.
The connotation is almost always neutral. It is a functional adverb used for qualification and accuracy.