To deteriorate is to worsen over time, to get progressively badder. It can be used with or without an object. The adjective form is deteriorative.
To decline can mean to get worse over time, a synonym of deteriorate. However, decline has other definitions that include becoming progressively smaller or fewer, which may be a good thing if one is talking about one’s weight. Another definition of decline is to reject, as in declining an invitation. It can mean the actual movement downward of an object or space, such as a hill having a steep decline or the moon declining to the horizon.
Decline can be a noun with the same spelling, while the adjective form is declinable. An alternate noun formation is decliner.
A note: Recline is a related word, but differs in that the object or person is supported and not falling all the way to the ground.
Examples
The UK evacuated its diplomatic staff from Yemen on Wednesday morning and shut its embassy as security continued to deteriorate across the war-torn country. [The Telegraph]
Despite spinal bifida’s deteriorative effects on her spine, Tatyana McFadden, who uses a wheelchair, won gold in the women’s 100 meters. [San Mateo Daily Journal]
The number of hate groups in the United States is on the decline for the second year in a row, according to an annual report released by the Southern Policy Law Center. [The Christian Science Monitor]
Molly Snyder, a spokesperson for Target, declined to comment other than to say the company has handed over the liquidation process to third parties. [Global News]
Frankly, there was no way to know that a guy who had demonstrably been declining over the last few years was, in fact, not going to magically become better again because he went back to Philadelphia. [Yahoo Sports]