In the context of gene regulation: transactivation is the increased rate of gene expression triggered either by biological processes or by artificial means, through the expression of an intermediate transactivator protein.
HIV and HTLV are just two of the many viruses that encode transactivators to enhance viral gene expression.
These transactivators can also be linked to cancer if they start interacting with, and increasing expression of, a cellular proto-oncogene.
The transactivator gene expresses a transcription factor that binds to specific promoter region of DNA.
If this specific promoter region is also attached to a reporter gene, we can measure when the transactivator is being expressed.