Necessary, essential for life or some process.
Elementary, simple, fundamental, merely functional.
Of or pertaining to a base; having a pH greater than 7.
Unremarkable or uninteresting; boring; uncool.
An elementary building block, e.g. a fundamental piece of knowledge.
A necessary commodity, a staple requirement.
Basic training.
Being an essential and permanent part of something.
Being an included feature that normally comes as an extra.
Constructed as a non-detachable part of a larger structure.
Anything (such as a piece of furniture, or a software feature) that is built in, not added as an extra.