...or just "island" (in the UK at least)

Zebra Crossing Without an Island

The two examples above illustrate zebra crossings that have no central island. This type of crossing should be seen as one crossing.

Zebra Crossing With an Island

s we can see in this example the crossing has an island so this crossing would be considered as two separate crossings. However, be very careful, as pedestrians will treat this as one crossing and usually walk straight across. They should walk to the central island and wait for the traffic to pull up.
http://www.driving-made-easy.co.uk/Zebra.htm

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Note added at 9 mins (2008-01-21 13:24:28 GMT)
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The "central island" is just a limited area, an island as the term says. But if it's a long paved stretch separating lanes, it's called "central reservation" in the UK, and, apparently, "median strip" in the US (according to reference below).

Definition here:
"On divided roads, including expressways, motorways, or autobahns, the central reservation (British English), median (North American English), median strip (North American English and Australian English) or central nature strip (Australian English) is the area which separates opposing lanes of traffic."

http://en.wikipedia.org/wiki/Central_reservation