At its most fundamental level, animation is about a relationship between two specific and separate properties, namely change on one hand and time on the other. Technically, animation defines change over time, that is, how a property adjusts or varies across time, such as how the position of a car changes over time, or how the color of a traffic light transitions over time from red to green. Thus, every animation occurs for a total length of time (duration), and throughout its lifetime, the properties of the objects will change at specific moments (frames), anywhere from the beginning to the end of the animation.
This definition is itself technical and somewhat dry, but relevant and important. However, it fails to properly encompass the aesthetic and artistic properties of animation. Through animation and through creative changes in properties over time, moods, atmospheres, worlds, and ideas can be conveyed effectively. Even so, the emotional and artistic power that comes from animation is ultimately a product of the underlying relationship of change with time. Within this framework of change over time, we may identify further key terms, specifically in computer animation. You may already be familiar with these concepts, but let's define them more formally.
Within an animation, time must necessarily be divided into separate and discrete units where change can occur. These units are called frames. Time is essentially a continuous and unbreakable quantity, insofar as you can always subdivide time (such as a second) to get an even smaller unit of time (such as a millisecond), and so on. In theory, this process of subdivision could essentially be carried on ad infinitum, resulting in smaller and smaller fractions of time. The concept of a moment or event in time is, by contrast, a human-made, discrete, and self-contained entity. It is a discrete thing that we perceive in time to make our experience of the world more intelligible. Unlike time, a moment is what it is, and it cannot be broken down into something smaller without ceasing to exist altogether. Inside a moment, or a frame, things can happen. A frame is an opportunity for properties to change—for doors to open, characters to move, colors to change, and more. In video game animation specifically, each second can sustain or contain a specified number of frames. The amount of frames passing within a second will vary from computer to computer, depending on the hardware capacity, the software installed, and other factors. The frame capacity per second is called FPS (frames per second). It's often used as a measure of performance for a game, since lower frame rates are typically associated with jittery and poor performance. Consider the following figure, showing how frames divide time:
Although a frame represents an opportunity for change, it doesn't necessarily mean change will occur. Many frames can pass by in a second, and not every frame requires a change. Moreover, even if a change needs to happen for a frame, it would be tedious if animators had to define every frame of action. One of the benefits of computer animation, contrasted with manual, or "old", animation techniques, is that it can make our lives easier. Animators can instead define key, or important, frames within an animation sequence, and then have the computer automatically generate the intervening frames. Consider a simple animation in which a standard bedroom door opens by rotating outwards on its hinges by 90 degrees. The animation begins with the door in the closed position and ends in an open position. Here, we have defined two key states for the door (open and closed), and these states mark the beginning and end of the animation sequence. These are called key frames, because they define key moments within the animation. On the basis of key frames, Unity (as we'll see) can autogenerate the in-between frames (tweens), smoothly rotating the door from its starting frame to its ending frame. The mathematical process of generating tweens is termed as interpolation. See the following figure, showing how frames are generated between key frames: