Nekoashi is a 'back stance' used in Karate. It means 'cat-feet' referring to the light placement and movement of the feet and is typically called 'cat stance' in english.

There are two main forms, a long stance, Mahanmi no nekoashi, and a short stance, Mashomen/Hanmi no nekosahi.

The long cat stance is effectively identical to Mahanmi no kokutsu dachi. It is used extensively in Kata for moves such as shuto uke.

The short cat stance is formed with 70-80% of the weight on the rear leg, which is directly below the body, foot turned about 30° from the direction the stance is facing. The front foot is slightly forward pointing straight in the direction of the stance, with the heel raised. Both knees are bent slightly and the body is either (Mashomen) full facing (the same direction as the front foot) or (Hanmi) half facing (about 45° off full facing, in the same general direction as the rear foot). Position of the arms is governed by the particular move being employed, but usually the rear fist is on the belt.
This stance too is also used a lot in Kata, for moves such as Sotouke (outer forearm block).

There's yet another cat stance namely Gyaku nekoashi (reverse cat stance - yes as in Gyakuzuki). It's a little hard to explain so bear with me. It's a front- or semi-front-facing stance, the choice depends on the specific move. The legs are both bent, but most of the weight (about 80%) is on the front leg which is directly below the body. The rear foot is positioned such that if you were standing with both feet together and then moved one back by about 1/2 to 1 foot length. Finally, the rear heel is raised and the back knee tucked in slightly behind the front. I've only seen this stance used for 'landings'. For example a large attack such as a big kick or jump in a kata often lands in a block in this stance. It's useful because it lends itself both to stepping into deep, strong stances (Zenkutsus) or twisting quickly into short, defensive stances (such as a Hanmi no nekoashi etc).