As mentioned, they're the same. Although "dhyana" can be translated as "meditation", it should be pointed out that at least in the yoga tradition, it refers to a specific, deeply focused state of one pointedness, like jhana, rather than being the act of performing a meditation technique itself - which is referred to as "dharana", I believe. So practicing "dharana" leads to "dhyana", in much the same way as particular techniques of concentration lead to "jhana".

They are the same, but in a different language. Jhana is Pāli, Dhyana is Sanskrit. Pāli ist the language of the oldest collection of discourses of the Buddha, on which Theravada buddhism is based. Sanskrit appeared later and is often used in Mahayana traditions.

As far as I know both Pāli and Sanskrit are artificially created languages, by merging various Indian dialects of the time, for the purpose of communicating spiritual/religious teachings across ethnic boundaries.

> Sanskrit appeared later and is often used in Mahayana traditions.

While Sanskrit was used in later Buddhist traditions, the term "dhyana" in Sanskrit predates the Buddha, as it was used in the Vedas, and a lot of Pali does seem to derive from Sanskrit (or at least, other local languages that were influenced by Sanskrit).

Now, "modern" Sanskrit and "vedic" Sanskrit can be considered two things (like English and Old English), but many of the terms used are the same, hence its influence on Pali that compare to modern Sanskrit terms.