noun
clandestine meeting; secret rendezvous; tryst
Typically used for lovers meeting in secret. The cross-reference 密会 is a more general term for secret meetings.
See also: 密会
二人は人目を避けて、しのび逢いを重ねた。
The two of them met secretly again and again, avoiding public attention.
あのカフェはしのび逢いの場所
That café is known as a place for secret rendezvous.
密会 is a broader term for any secret meeting, not necessarily romantic. しのび逢い strongly implies a lovers' tryst.
Compound of 忍び (しのび, stealth) and 逢い (あい, meeting). The kanji 逢 specifically suggests a romantic or intimate encounter.