【Delay】 , 【retard】 , 【slow】 , 【slacken】 , 【detain】 are not always close synonyms, but they carry the same basic meaning: to cause someone or something to be behind in his or its schedule or usual rate of movement or progress.

【Delay】 implies the operation, usually the interference, of something that keeps back or impedes, especially from completion or arrival at a set or given time.

【Retard】 applies especially to motion, movement, or progress and implies something which causes it to reduce its speed.

【Slow】 (often followed by up or down) and 【slacken】 also imply a reduction in speed or rate of progress, but 【slow】 usually implies deliberation or intention and 【slacken】 , an easing or letting up or a relaxation of some sort.

【Detain】 (see also ARREST 2) implies a being held back beyond an appointed time, often with resulting 【delay】 in arrival or departure or in accomplishment of what one has in mind.