What is the term used when separate items are joined together into one place without changing them?

The term that best describes the process of joining separate items together into one coherent entity without altering the individual items is concatenation. Concatenation typically refers to linking things in a series or chain, commonly used in programming when combining strings or sequences. For example, if you have two strings, "Hello" and "World", concatenating them would result in "HelloWorld", maintaining the integrity of each original string while creating a new unified output.

In contrast, aggregation involves collecting items into a whole but may imply some transformation or summarization, while combination can imply mixing elements which might change their original state. Modification, on the other hand, inherently suggests altering the items involved. Thus, concatenation is the precise term that captures the essence of uniting items without any change.