Срок действия истёк.

A very literal breakdown is:

• срок = term, period, deadline
• действия = of functioning / of validity / of being in effect
• истёк = expired, ran out, elapsed

So the whole phrase is literally something like the period of validity has expired or the term of effect has run out.

In natural English, this is often rendered as:

• The validity period has expired
• It has expired
• The expiration date has passed
  depending on context.

Because срок often takes a dependent noun in the genitive to show the period of what.

So:

• срок действия = period of validity / term of operation
• literally, term of action/effect

This is a very common Russian pattern:

• срок годности = shelf life / expiration period
• срок аренды = lease term
• срок службы = service life

So действия is genitive singular of действие, because it depends on срок.

Срок действия is a fixed expression meaning period of validity or time during which something remains valid/in force.

It is commonly used for things like:

• passports
• visas
• contracts
• certificates
• bank cards
• tickets
• documents in general

So if you see Срок действия истёк on a document or in a system message, it usually means the document, card, code, or offer is no longer valid.

Истёк is:

• past tense
• masculine singular
• perfective verb form

Its infinitive is истечь.

Here it agrees with срок, which is a masculine singular noun.

So the grammar is:

• срок — masculine singular subject
• истёк — masculine singular past tense verb

If the subject were feminine, neuter, or plural, the form would change:

• лицензия истекла́ = the license expired
• время истекло́ = the time expired / ran out
• сроки истекли́ = the periods expired

Because the true subject of the sentence is срок, not действия.

In срок действия, the main noun is срок.
Действия only describes it: the period of validity.

Russian past tense verbs agree with the subject, so the verb matches срок:

• срок = masculine singular
• therefore истёк = masculine singular

This is a very common point of confusion for learners, because the noun right next to the verb is not always the grammatical subject.

The correct spelling is истёк with ё.

The letter ё always carries stress, so истёк is pronounced roughly is-TYOK.

However, in real Russian writing, especially in print, ё is often replaced by е, so you may often see:

• Срок действия истек

Even when written with е, native speakers still understand it as истёк.

A natural pronunciation is:

Срок де́йствия истёк.

Stress:

• срок — one syllable
• де́йствия — stress on the first syllable
• истёк — stress on ё

A rough English-style approximation:

srok DEY-stvi-ya is-TYOK

But it is best to listen to native audio if possible, especially for:

• the rolled or tapped р
• the reduced unstressed vowels
• the ё sound in истёк

Yes, it sounds fairly formal and standard. It is very common in:

• official notices
• forms
• websites
• documents
• software messages
• legal or administrative contexts

For example, you might see it referring to:

• a passport
• a bank card
• a session token
• an offer
• a license

In everyday speech, people might also say simpler things depending on context, such as:

• Он уже недействителен = It is no longer valid
• Срок уже прошёл = The time period has already passed
• Она просрочена = It is expired / overdue

But Срок действия истёк is perfectly normal and very common.

Yes. Russian word order is more flexible than English.

You can say:

• Срок действия истёк
• Истёк срок действия

Both are grammatical.

The version with Срок действия истёк is the most neutral and straightforward.
Starting with Истёк can sound a bit more literary, emphatic, or context-dependent.

In notices and labels, the neutral order is more common.

Yes, sometimes. Срок истёк means the term has expired / the deadline has passed.

But it is less specific.

• Срок действия истёк clearly means the validity period expired
• Срок истёк could mean the deadline or allotted time expired

So if you are talking about a document, card, pass, or contract, срок действия is often better because it tells you exactly what kind of period is meant.

They are related, but not the same.

• срок действия = validity period, period during which something is officially valid or in force
• срок годности = shelf life, expiration period for something fit to use or consume

Examples:

• passport, visa, contract, certificate, card → usually срок действия
• food, medicine, cosmetics → usually срок годности

So:

• Срок действия истёк = the document/card/etc. is no longer valid
• Срок годности истёк = the food/medicine/etc. has expired

Sometimes yes, but истёк is more standard in this specific expression.

Compare:

• Срок действия истёк = very standard, official, set phrase
• Срок действия закончился = understandable, but less idiomatic in formal usage

Russian often prefers истёк with things like:

• срок
• срок действия
• срок годности
• срок давности

So for learners, it is best to remember срок ... истёк as a common pattern.

You would normally use forms of the same verb:

• истекает = is expiring / expires
• истечёт = will expire

Examples:

• Срок действия истекает завтра. = The validity period expires tomorrow.
• Срок действия истечёт через месяц. = The validity period will expire in a month.

So the sentence you have, Срок действия истёк, is specifically has expired / expired.

Russian has no articles like a or the.

So Срок действия истёк can mean:

• The validity period has expired
• A validity period has expired

In practice, the context usually makes it definite in English, so translators often choose the.

That is normal when going from Russian to English: Russian often leaves definiteness unstated, while English requires you to choose.

Yes. It is a complete Russian sentence:

• subject: срок действия
• predicate: истёк

Russian often expresses things very compactly, especially in notices and official language. Nothing is missing grammatically.

That shortness is one reason it may look abrupt to an English speaker, but it is completely natural Russian.