Variant characters are often called 异体字.
How about words? E.g.:
歺厅 is a variant of 餐厅
[Specifically looking for XX的XX, i.e.: 餐厅的XX]
Words variants in Chinese are 异形词.
For example, there is a document published by the govenment of PRC, 《第一批异形词整理表》(The First Series of Standardized Forms of Words with Non-standardized Variant Forms).
However, I think 歺厅/餐厅 is not a typical example of “异形词”.
In fact, 歺 is one of the second round of simplified Chinese characters (第二批简化字, 二简字), which is obsolete and non-standard now. The different between 歺厅 and 餐厅 is on the character, not the word. So in this example we may say:
Better examples of “异形词” can be:
I am afraid they should not be considered as 「异形词」.
「异形词」 are multisyllabic words with the same pronunciation but written in different graphemes. The case of 「餐厅」 and 「歺厅」is not the same as the case of 「唯一」 and 「惟一」 or the pair 「按语」 and 「案语」, for 「唯」、「惟」、「按」、「案」are separated graphemes (all in-use standard Chinese characters), but 「歺」 is an abandoned variant of the standard character 「餐」, and the two should be considered as the same grapheme (but different glyphs).
To be clear:
「按」: press 「案」: table/desk/case
They are different graphemes.
→「按语/案语」: editorial comment
Exactly the same pronunciation and meaning but written in two forms consist of characters with different meanings. They are 异形词.
「餐」: meal 「歺」: meal
They are different glyphs but the same grapheme. 「歺」is the abandoned simplified form of 「餐」.
We often refer the two as 「异体字」, and of course, this term is only used to refer different glyphs, never multisyllabic words or phrases.
So the sayings:
「『歺』是『餐』的异体字/二简字。」 - Correct
「『歺厅』是『餐厅』的异形词。」 - Wrong
It's better to say, 「『歺厅』中『歺』是『餐』的异体字。」