The phrase "in decline" is often used to describe a decrease or diminishing of something over time. There are many synonyms for this phrase that can be used to express the same idea in different ways. Some examples include waning, diminishing, dwindling, fading, ebbing, decreasing, and falling. These terms can all be used to describe a range of situations, from the decline of a business or economy to the fading of a relationship or the dwindling of a natural resource. Each of these words carries its own nuances and connotations, allowing writers and speakers to choose the one that best fits their intended meaning and tone.