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LingakuLingakuNote: 以下 includes the stated value (≤). Contrast with 未満 which excludes it (<).
以下（いか） is used after a number, quantity, or noun to mean "X or below", "X or less", or "not exceeding X". It indicates that something is at or under a certain limit.
When attached to a number, 以下 means that number and everything below it (i.e., the stated number is included).
This is an important distinction: いか以下 includes the number itself, whereas みまん未満（みまん） means "less than" and excludes it. For example, じゅう はっさい１８歳いか以下 means "18 and under," while じゅう はっさい１８歳みまん未満 means "under 18 (not including 18)."
以下 is used in rules, regulations, and requirements to set upper limits.
以下 can also follow nouns to mean "below the level of" or "inferior to", sometimes used for emphasis or comparison.
In written or formal contexts, 以下 can also mean "the following" or "as follows", referring to what comes after.
The counterpart of 以下 is いじょう以上（いじょう）, meaning "X or more / X and above." Together they are used to define ranges.