A single piece of data in an <apex:pageBlockSection> that takes up one column in one row. An <apex:pageBlockSectionItem> component can include up to two child components. If no content is specified, the column is rendered as an empty space. If one child component is specified, the content spans both cells of the column. If two child components are specified, the content of the first is rendered in the left, "label" cell of the column, while the content of the second is rendered in the right, "data" cell of the column.

Note that if you include an <apex:outputField> or an <apex:inputField> component in an <apex:pageBlockSectionItem>, these components do not display with their label or custom help text as they do when they are children of an <apex:pageBlockSectionItem>. Also note that <apex:pageBlockSectionItem> components cannot be rerendered; rerender the child components instead.