'Bidirectional' is a generic term used to describe software products that support Arabic and other languages that are written right-to-left. More specifically, bidirectional refers to the product ability to manipulate and display text for both left-to-right and right-to-left languages. For example, to display a sentence containing words written in both English and Arabic requires bidirectional capability.
RTL is frequently used as an abbreviation for right-to-left. Similarly, LTR is frequently used as an abbreviation for left-to-right.