The term "peak purchase" can be synonymous with a few different phrases depending on the context. It could be described as the highest amount of purchases made during a particular period, which could also be referred to as the "pinnacle of sales" or "zenith of buying." Alternatively, if discussing a specific product or service, "peak purchase" could be interchanged with phrases such as "most popular purchase" or "bestselling item." Another potential synonym could be "peak demand," which would describe the time or period when a particular product or service is in highest demand. Ultimately, the exact synonym for "peak purchase" will depend on the specific context in which it is used.