The Indefinite Specific Pronoun: один
You already know один as the number "one." But it frequently acts as an indefinite pronoun meaning "a certain." You use it when you have a specific person or object in mind, but you choose not to explicitly name them.
| A certain famous entrepreneur works in this business sphere. | Один известный предприниматель работает в этой деловой сфере. |
Like any adjective or pronoun, один must agree with the noun it describes in gender, number, and case. For a neuter noun like знакомство (acquaintance), use одно.
| I have a certain important acquaintance in this sphere. | У меня есть одно важное знакомство в этой сфере. |
When attending networking events, you will often use the verbs знакомить (imperfective) and познакомить (perfective) to introduce people. You introduce someone (accusative) to someone else (с + instrumental). Naturally, один declines to match these cases.
| Tomorrow I plan to introduce you to a certain investor. | Завтра я планирую познакомить вас с одним инвестором. |
This pronoun even has a plural form: одни (meaning "some" or "a certain group of"). Just like the singular forms, it declines to match the case of the noun.
| Our director often likes to introduce his team to some promising sponsors. | Наш директор часто любит знакомить свой коллектив с одними перспективными спонсорами. |
Because один can mean "one", "alone", or "a certain", context is key. You might even see it playing two different roles in the exact same sentence!
| A certain manager came to the meeting alone, and I decided to introduce him to the colleagues. | Один менеджер пришёл на встречу один, и я решил познакомить его с коллегами. |