For the geographical feature:
* Alluvial fan (especially if it's a fan-shaped delta)
* Estuary (if it's a delta at the mouth of a river)
* Mouth (of a river)
* Outwash plain (if it's formed by glacial meltwater)
* Plain
* Lowland
* Floodplain
* Wetland
For the mathematical concept:
* Difference
* Increment
* Change
* Variation
For the Greek letter:
* Triangle (though technically this refers to the shape of the letter, not the letter itself)
For other meanings:
* Change (as in "a delta in the stock market")
* Variation
* Difference
It's important to choose the synonym that best fits the specific meaning of "delta" in your context.