Can I define a variable that could be used by another signal

As per the subject. For example I have 2 signal in my strategy: 1) LongEntry, 2) stoploss. In LongEntry, I called Buy next bar at market;, In stoploss, I called setstoploss(amt); where amt is calculated in LongEntry signal.

You can't since all variables have file scope. However, you can communicate from one signal to another via global variables.

As per the subject. For example I have 2 signal in my strategy: 1) LongEntry, 2) stoploss. In LongEntry, I called Buy next bar at market;, In stoploss, I called setstoploss(amt); where amt is calculated in LongEntry signal.

Look up:

i_setplotvalue
i_getplotvalue