In the context of DNA, an **alien** would refer to genetic material or sequences that are not native or typical to the species being studied. This could include:

- **Horizontal Gene Transfer**: Genes that have been transferred between different species, which is not uncommon in bacteria but can also occur in other organisms.

- **Genetic Engineering**: DNA sequences intentionally introduced into an organism from another species, often for biotechnological purposes like creating transgenic organisms.

- **Extraterrestrial DNA**: Hypothetically, if DNA from an extraterrestrial life form were to be discovered, it would be considered alien in the most literal sense, as it would not originate from Earth's biosphere.

In all these cases, "alien DNA" would describe sequences that are foreign to the genome they are found in, either naturally introduced or through human intervention.