In Aikido, what are the kanji characters for "internal strength"? What is the definition of internal strength?
In Chinese it's 內勁, where I think the better translation is internal force. In Taiji Quan we say at a fundamental level, fighting is about generating a force (not a strength) and affect your opponent with it. There are many different types of forces, each with its own unique set of characteristics - amount, speed, rate of acceleration, duration, timing, direction, store and release process, etc.
Just as there is no one best car, there is no one best type of force, it all depends on what you're trying to do. In Chinese martial art, we classify forces into two broad categories: external and internal. External means from observing the physical movement used to generate that force, you can understand all its characteristics, and deal with it accordingly. Internal refers to ones where the physical movement is so small, so hard to observe, you can only understand it by feeling it. In general you can generate greater amount of forces with external force, whereas with internal force the chief advantage is that they are great for controlling the opponent.
Lots of so called external martial art actually uses internal type of forces too, but usually they don't emphasize it, or make so little use that there's no need to categorize it and study in detail to start with (ex. boxing). Whereas in something like Taiji Quan (and maybe Aikido?), while we do use external force to finish opponent off, the emphasis is on using internal force to unbalance the opponent first. In a complete art we should understand and use all types of forces as appropriate.
The reason I don't like the term "strength", besides it's not the most accurate translation, is that it further feeds into some of the fundamental misconceptions about internal force. When we say strength, of course more is better. But actually, in training we want to develop as much as possible, in fighting use as little as possible. Be efficient is not some nice-to-have thing when you're fighting multiple opponents or have to fight for a long duration. The other misconception being we want internal force because it's something even more powerful (quantity wise) than external force, when in fact we're comparing apples and oranges.
Here's an article that covers the basics on Taiji Quan's approach to use of force: http://www.ycgf.org/Articles/TJ_Jin/TJ_Jin1.html