What is the definition of straight and level-flight?

I assume you're referring to an aircraft.

Straight and level flight is achieved in an aeroplane, glider or airborne vehicle when lift is approximately equal to drag - eg. The craft is not rising or descending, providing the 'level flight'. In addition, the roll of the aircraft is usually neutral, providing the 'straight', and thrust is approx. equal to drag to maintain a constant speed.