In the eyepiece catalog section of this website, we use various terms to describe our subjective opinion of the performance of the individual eyepieces we carry. These terms are described below. 

Number of lens elements: typically, more lenses in an eyepiece mean sharper images, and/or a wider field of view, and/or longer eye relief . . . although sometimes with a loss of contrast compared to simpler eyepieces.

Sharpness: how well the eyepiece concentrates the light of a star into a point at the center of the field; how crisply defined lunar and planetary details appear; how cleanly binary stars are split.

Astigmatism: an aberration that turns stars into fuzzy oblongs instead of points, particularly towards the edge of the field. The oblongs point towards the center of the eyepiece field on one side of focus, but are at right angles to the center on the other side of focus. Not to be confused with field curvature, below.

Color correction: how free an eyepiece is from colored halos around stars at the edge of the field, false color in planetary images, or stars that change color as they move from the center to the edges of the field.

Ghosting: a flare of unwanted light around bright objects, or faint multiple images of bright objects, due to internal eyepiece reflections.

Field curvature: an inability to bring the center and edge of the field into focus at the same time, with the edge out of focus with the stars bloated when the center is sharply focused and vice-versa. Not to be confused with astigmatism, above, in which the stars change into ovals at the edge of the field.

Distortion: unequal magnification across the eyepiece field, so that straight lines appear curved; it is barrel distortion if the centers of straight lines bow outwards, and pincushion distortion if the centers bow inwards; excessive distortion can produce an annoying"seeing through a goldfish bowl" effect when panning across a star field. 
Contrast: the range from light to dark seen in an eyepiece; high contrast makes the shadows in the bottom of lunar craters black, aids in splitting close binary stars,enhances subtle lunar and planetary detail, and helps ferret out small and faint planetary nebulas against a truly dark sky background; low contrast (due to internal reflections within the eyepiece lenses caused by unblackened lens edges and lesser-quality anti-reflection coatings) shows lunar crater shadows in shades of gray, rather than stark black, and makes subtle planetary and nebula detail more difficult to see.