blast-off or lift-off

27. blast off, (verb phrase) a. (of a rocket) to leave a launch pad under its own power.
http://dictionary.reference.com/browse/blast+off?s=t

lift off (noun) 1. Aeronautics, Rocketry .
a. the action of an aircraft in becoming airborne or of a rocket in rising from its launching site

I want to know the difference when used with regard to spaceship's leaving.

They both refer to the instant in time that a spaceship or rocket leaves the launch site.
If you read the definitions, blast off is a verb, and lift off a noun.
Do you know the difference between a verb and a noun?

According to my three dictionaries, both blast-off and lift-off can be nouns.

Then you can use them interchangeably.. If the definitions are identical, then there is no difference.

Blastoff means there is a blast, and then “off”, meaning a blast of propellants, than, the rocket goes off. What happens after the blast? To demonstrate this, click a pen on a table twice. Hold it and then let go. What happens? The pen falls after going a few inches. The blast with the pen is when you clicked it, and the off is when you let go.

Liftoff is when the rocket lifts of