(used with a negative) not even; not any — an emphatic negative meaning (e.g. “not even one day”).
彼は一日たりとも休まないで働き続けた。
He continued working without resting for even a single day.
一つたりとも無駄にできない材料だ。
This is material you cannot waste even a single piece of.
あの事件については一秒たりとも忘れてはいけない。
You must not forget that incident for even a second.
彼女は一瞬たりとも油断しなかった。
She didn't let her guard down for even a moment.