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.
essential; extremely important
Related to a foundation. base, or basis; serving as a foundation.
The lowest frequency of a periodic waveform.
A main or major principle, rule, law, etc. which serves as the foundation or basis of a system; an essential part
The lowest partial of a complex tone.