Aspect ratio is the proportional relationship between the width and height of a video frame, expressed as two numbers separated by a colon. The 16:9 widescreen aspect ratio is the standard for modern live streaming and most displays, while 4:3 is the older standard definition ratio still encountered in legacy systems. The aspect ratio affects how footage fills a screen and must be consistent between capture, encoding, and display to avoid stretching or letterboxing in live stream output.