In the field of molecular biology, trans-acting (trans-regulatory, trans-regulation), in general, means "acting from a different molecule" (i.e., intermolecular).
It may be considered the opposite of cis-acting (cis-regulatory, cis-regulation), which, in general, means "acting from the same molecule" (i.e., intramolecular).
In the context of transcription regulation, a trans-acting factor is usually a regulatory protein that binds to DNA.
[1] The binding of a trans-acting factor to a cis-regulatory element in DNA can cause changes in transcriptional expression levels.
microRNAs or other diffusible molecules are also examples of trans-acting factors that can regulate target sequences.