secret; clandestine; confidential; arcanum;
例句：
1.我的秘密武器一直是我的家庭。
My secret weapon was my family always.
2.许多基督徒是在家里秘密集会的。
Many christians were meeting secretly in homes.
3.facebook和其他社交网络已有数百万易受伤害的、秘密的低龄用户。
Facebook and other social networks already have millions of vulnerable, clandestine underage users.
4.此次暗杀中，27个秘密特工使用伪造的护照出行。
The killing involved 27 undercover agents travelling on forged passports.
5.正因如此，一些秘密将继续保持下去。
So we are left with something of a mystery.