A limiter is basically a compressor that works at high ratios. There is no set rule as to what the ratio needs to be for it to stop being a compressor and start being a limiter. Some suggest 10:1, other 20:1, and others something else altogether. The high compression ratio is often combined with a fast attack to stop any sound getting past the set threshold. This is often useful for stopping clipping to occur. When the limiter is set to behave this way (i.e. stop the entire signal from reaching a set threshold) it is usually referred to as a “brick wall limiter”.