1) Base 2) Infrastructure 3) Understructure 1) Construction terminology 2) Infrastructure 3) Lowest support of a structure 4) Structure 
In mathematical logic, an (induced) substructure or (induced) subalgebra is a structure whose domain is a subset of that of a bigger structure, and whose functions and relations are the traces of the functions and relations of the bigger structure. Some examples of subalgebras are subgroups, submonoids, subrings, subfields, subalgebras of algebras...
• (n.) Same as Substruction. • (n.) An under structure; a foundation; groundwork.
A tissue or structure wholly or partly beneath the surface. ... (05 Mar 2000) ... 
(sub´struk″chәr) the underlying or supporting portion of an organ or appliance.
the foundation on which the derrick or mast and usually the drawworks sit. It contains space for storage and well-control equipment. 
Sub·struc'ture noun [ Prefix sub- + structure .] 1. (Architecture) Same as Substruction . 2. An under structure; a foundation; groundwork. 
Type: Term Pronunciation: sŭb′strŭk-chūr Definitions: 1. A tissue or structure wholly or partly beneath the surface.
a structure forming the foundation of a building or other construction. · the foundations, piers, and abutments upon which the trusses or girders of the spans of a bridge rest. · any basic structure or organization.
the basic structure or features of a system or organization