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.  
In the context of receptor signaling, transactivation occurs when one or more receptors activate yet another; receptor transactivation may result from the crosstalk of signaling cascades or the activation of G protein–coupled receptor hetero-oligomer subunits, among other mechanisms.