What is the term for a word which can be written with one kanji or two kanji but where the meaning is the same?


I'm asking mostly about verbs like 陥る / 落ち入る though I'd also be interested in
nouns and adjectives.

I'm not really looking for synonyms, more words where the meaning and
pronunciation are the same but the kanji changes. I know there are instances
of this though I don't know what it's called and ideally I'd like some more
examples.




One convenient way to manage situations like this is to combine following
pieces to convey your intention:

  * 同音【どうおん】 same reading (on- or kun-)
  * 同訓【どうくん】 same kun-reading
  * 異音【いおん】 different reading
  * 同義【どうぎ】 same meaning
  * 異義【いぎ】 different meaning
  * 類義【るいぎ】 similar meaning
  * 同字【どうじ】 same kanji
  * 異字【いじ】 different kanji

For example, there are
[同音異義語](http://ja.wikipedia.org/wiki/%E5%90%8C%E9%9F%B3%E7%95%B0%E7%BE%A9%E8%AA%9E),
[同音異字語](http://ja.wikipedia.org/wiki/%E5%90%8C%E9%9F%B3%E7%95%B0%E5%AD%97),
[同字異音異義語](http://www.h-dc.com/hantai/ionigigomain.htm), etc.

You can mix these freely, and at least make yourself understood in blog posts
or Q&A forums like _Y! Chiebukuro_. But professional linguists may have better
specific words or rules which I'm not aware of.

Your 陥る/落ち入る pair is 同音異字 which happens to be 同義, too.

