The correct term that describes the structural relationship between cis-1,3-dimethyl cyclopentane and trans-1,3-dimethyl cyclopentane is diastereomer.

Explanation:
- Diastereomers are a type of stereoisomers that have different spatial arrangements of atoms, but are not mirror images of each other.
- In the case of cis-1,3-dimethyl cyclopentane and trans-1,3-dimethyl cyclopentane, they both have the same molecular formula and connectivity of atoms, but differ in the arrangement of substituents around the double bond.
- The cis isomer has the two methyl groups on the same side of the double bond, while the trans isomer has them on opposite sides.
- Since the cis and trans isomers are not mirror images of each other and have different spatial arrangements, they are classified as diastereomers.

To better understand the difference between enantiomers, diastereomers, geometric isomers, and regioisomers:

- Enantiomers: Enantiomers are a type of stereoisomers that are non-superimposable mirror images of each other. They have the same connectivity of atoms but differ in their spatial arrangement due to the presence of a chiral center.
- Geometric isomers: Geometric isomers, also known as cis-trans isomers or E-Z isomers, occur when there is restricted rotation around a double bond or a cyclic structure. They have the same connectivity of atoms but differ in the arrangement of substituents around the double bond or within the cyclic structure.
- Regioisomers: Regioisomers occur when there is a different connectivity of atoms within a molecule. They have the same molecular formula but differ in the arrangement of atoms within the molecule.

In summary, since cis-1,3-dimethyl cyclopentane and trans-1,3-dimethyl cyclopentane have the same connectivity of atoms but differ in the arrangement of substituents around the double bond, they are classified as diastereomers.