John 4:48
Jesus said to him, "Unless you people see signs and
wonders, you will not believe."
"Teras" means something very unusual, out of the ordinary,
amazing, or astonishing. 
"Dunamis".  A "power".  It is sometimes used of ordinary
power, but is often translated "miracle".  It is often used in
combination with "semeion" and "teras". 
"Dunamis"  emphasizes the source of the event.