retrograd

Retrograd is an adjective used in several languages to denote backward, reversed, or opposite in direction. It derives from Latin retrogradus and is common in Germanic languages such as German, Danish, Norwegian, and Swedish, where the spelling retrograd is used alongside or instead of retrograde.

In astronomy, retrograd motion describes the apparent backward movement of a planet across the sky relative to distant stars. This occurs when Earth overtakes another planet in its orbit or when their relative positions create the illusion of backward travel. A planet can appear to loop or pause at a retrograde point before resuming its usual eastward motion.

In music, retrograde is a technique in which a melodic line is presented backwards, from end to beginning. It is a standard device in Renaissance music and remains central in 20th-century serial and thematic development, where a tone row or motif may be performed in retrograde as a permutation.

In science and medicine, retrograd describes processes that run in the opposite direction. Examples include retrograde ejaculation (where semen enters the bladder rather than exiting through the penis), retrograde transport of substances within cells, and retrograde amnesia (loss of memories formed before an event). In food science, retrogradation refers to the slow recrystallization of gelatinized starch as it cools, which can affect texture and shelf life.

In other contexts, retrograd appears in general discussions of backward movement or reversal in languages and disciplines, and is often used interchangeably with retrograde in languages that spell the term as retrograd. The meaning is typically governed by the field-specific usage.