variant character; variant form of a character
Refers to a character that is an alternate form of another, especially in kanji. Often used in discussions of orthography, dictionaries, and character encoding.
"渕" is a variant character of "淵".
This kanji dictionary also lists variant characters.
新字体 is the simplified modern form of a kanji, whereas 異体字 can refer to any variant, not necessarily the simplified one.
Compound of 異体 (itai, 'different form') + 字 (ji, 'character'). The term is a direct calque from Chinese 異體字 (yìtǐzì).