I Want to define some tags as a synonyms for other tag. When user search for any tag it will get result for both tag. Like we have hairfall and falling hair two tags. I need to define that hairfall is a synonyms for falling hair. So whenever user search with a keyword hairfall or falling hair. He will get result for related to both tags.
If anyone knows the module for the same task please share it with me.
Maybe the Synonyms module is what you are looking for. It provides:
per vocabulary settings - now you are able to enable/disable functionality on per-vocabulary basis;
field independency - you can choose what fields of term should be considered synonyms. Now it's not only a text field, but entity reference fields and others. See below for detailed explanation.
search integration - as this module originally was created for enabling node search by synonyms of the terms nodes reference to, this feature is still present in the 7th version;
synonyms friendly autocomplete widget - module ships with its own widget for taxonomy term reference field types, which supports autocomplete suggestions based not only on the names of terms but also based on their synonyms.