Truncation is like a wildcard. Added to the stem of a word, it will find that stem plus anything that comes after it. The symbol used to truncate a word depends upon the index, database, or Web search engine you are using.

psychol? will return records on psychology, psychological, psychologist.
? is used as the truncation symbol, for example, in ABI/Inform and WestCat.
environ* will return records on environment, environments, environmental.
* is used as the truncation symbol in Wilson Business Abstracts and Periodical Abstracts.