All prayer is meditation, but not all meditation is prayer. A cynical understanding is that prayer is religiously-sanctioned meditation. Meaning you’ll get looked down on by religious sorts if you describe a way of praying that they consider to be ‘mere’ meditation.
Christian churches used to warn against meditation considering it one step short of idolatry. When you sit in divine contemplation, these people consider it very important what’s going on in your mind. It’s the sort of moralism that drove people to gnosticism or even paganism.
In my experience, I’ll say a meditation is prayer when it’s particularly “Christian.” Meaning it centers around Jesus and has the focus on redemption / salvation. Particularly spiritual Christians are basically gluttons for that feeling, that “I was lost and now I’m found” feeling, and can call it up pretty much daily. These are the sorts you find in born-again traditions. I get a taste for it maybe once a season or so.