Introduction:
An emulsion is a mixture of two immiscible substances, typically a liquid dispersed in another liquid. Emulsions are formed when one liquid is dispersed as small droplets throughout the other liquid. Emulsions can be classified into oil-in-water (O/W) or water-in-oil (W/O) based on the nature of the dispersed phase and continuous phase.

Explanation:
Among the given options, shaving cream is not an emulsion. Here's why:

Butter:
- Butter is an emulsion made by churning cream or milk.
- It consists of water droplets dispersed in a continuous fat phase.
- The water droplets are the dispersed phase, and the fat is the continuous phase.
- Therefore, butter is an example of an O/W emulsion.

Milk:
- Milk is also an emulsion, specifically an O/W emulsion.
- It consists of tiny fat globules dispersed in a continuous aqueous phase.
- The fat globules are the dispersed phase, and the water is the continuous phase.
- Homogenization is used to break down the fat globules into smaller sizes, resulting in a stable emulsion.

Shaving Cream:
- Shaving cream is not an emulsion.
- It is a foam, which is a dispersion of gas bubbles in a liquid or solid.
- The gas bubbles in shaving cream are responsible for its light and fluffy texture.
- It does not contain two immiscible liquids like an emulsion.

Face Cream:
- Face creams are commonly emulsions used for skincare.
- They are typically O/W emulsions, with water as the continuous phase and oils as the dispersed phase.
- Other ingredients such as emulsifiers and stabilizers are added to ensure stability and prevent separation.
- These emulsions provide moisturization and nourishment to the skin.

Conclusion:
Among the options given, shaving cream is not an emulsion. It is a foam, whereas butter, milk, and face cream are all examples of emulsions. Emulsions play a significant role in various industries, including food, cosmetics, and pharmaceuticals, where the stability and dispersion of two immiscible liquids are required.