Short for Standard Output Sensitivity or Standard Output Specification. The average level in an sRGB image must be 18 percent gray (plus or minus 1/3 stop), with no exposure compensation, when exposed according to the specification. The output level is measured in the sRGB output from the camera so it is only applicable to sRGB images (JPEG, typically) and not raw image format. Not applicable with multi-zone metering.
Canon glossary us