△的含义有时有差值的意思，比如△t为时间差，请问△在英文里怎么讲，用哪个单词表示？谢谢

Delta

Delta most commonly refers to:

1 Delta (letter), the letter Δ or δ in the Greek alphabet, also used as a mathematical symbol

2 River delta, a landform at the mouth of a river

you are asking about the 1st one.