Question and Item are often used interchangeably, and this is typically fine. One item can contain multiple questions, but the majority of items have a single question, making the two terms roughly equivalent in most cases.

Question: a prompt and interaction for submitting a response.

Item: a combination of one or more questions and zero or more passages, and associated metadata.

Metadata is the important difference because it makes much of search and reporting possible. As only items can have metadata, it is recommended to author more small items than fewer large items.