忆 is a bound form (b.f.) — it cannot stand alone as a word. You'll only see it as part of compounds like 回忆 (huíyì, to recall), 记忆 (jìyì, memory), or the chengyu 忆苦思甜 (yì kǔ sī tián, recall past bitterness to appreciate present sweetness). Think of it as a literary/formal morpheme for 'remember' that always attaches to another character.