Underpinning (noun)

1. The action of providing support or foundation for something.
2. A foundation or support upon which something is built.

Origin:

Late 19th century: from under- (expressing sub-) + pinning.

Examples:

1. The underpinning of the bridge was reinforced to withstand the weight of the traffic.
2. The underpinning of the theory was based on solid research.
3. The underpinning of the argument was a series of well-reasoned premises.
4. The underpinning of the building was damaged during the earthquake.
5. The underpinning of the relationship was trust and mutual respect.