Eccentricity refers to the shape of the orbit, derivable from the highest and lowest distances in the orbit of the orbiting body (there's actually a bunch of ways to calculate it that are mathematically equivalent). It's related to modeling orbits as conic sections. An eccentricity of 0 is a perfect circle, <1 is a normal elliptical orbit, >=1 is an escaping trajectory.

For example, Earth's orbit around the sun is ~0.0167, Pluto's is 0.248.