Bidrectional appears to refer to bidirectional processing, where a system evaluates both the text before and after a target section. In language modeling, this allows surrounding context on both sides to inform interpretation or prediction, unlike unidirectional systems that use only preceding context.
A term used to describe a system that evaluates the text that both precedes and follows a target section of text. In contrast, a unidirectional system only evaluates the text that precedes a target section of text.