Cultural diffusion is the spread of culture, including aspects such as clothing and food, from one group to another, typically as a result of making contact for the first time. European explorers bringing back foods and other items, such as tobacco, from newly explored lands is an example of cultural diffusion.
When one culture begins to adopt elements of another, be it in manner, religion, food, clothing, agricultural practice or other cultural elements, this is cultural diffusion. People in Moscow experienced cultural diffusion from America when McDonalds was first introduced in that region. The concept of a fast-food restaurant serving hamburgers was exotic and new to them at that time. Cultural diffusion isn't necessarily a good thing, however. It can lead to the displacement of native cultural traditions, and it can even be dangerous. For example, according to Boise State University, when French explorers arrived in North America, they introduced the smallpox virus, which ended up devastating populations of natives whose immune systems had never been exposed to that particular disease.