Термін дії короткий.

Literally, it is something like term of action/effect.

In real usage, термін дії means the period during which something is valid or in force. You will often see it with things like:

• паспорт = passport
• віза = visa
• договір = contract
• ліцензія = license

So it is a normal administrative phrase for validity period.

Because дії is the genitive singular form of дія.

Ukrainian often uses the pattern:

noun + genitive noun = the X of Y

So here:

• термін = term, period
• дії = of validity / of operation / of effect

You can think of it as answering the question:

• термін чого? = the term of what?
• дії = of validity/effect

This is a very common structure in Ukrainian.

Yes, дія often means action, but it also has broader meanings such as:

• effect
• operation
• functioning
• being in force

In official or practical language, that broader meaning is very common. So in термін дії, it is not talking about a physical action; it is talking about the period during which something applies, works, or remains valid.

Because Ukrainian has no articles.

There is no direct equivalent of a/an or the in normal Ukrainian grammar. Whether something is definite or indefinite is usually understood from context.

So Термін дії короткий can mean:

• The validity period is short
• or, depending on context, A validity period is short

In most real situations, the context tells you which one is meant.

In the present tense, Ukrainian usually omits the verb to be.

So:

• Термін дії короткий.
  literally: Validity period short.
  natural English: The validity period is short.

This is completely normal.

Compare with other tenses, where the verb does appear:

• Термін дії був короткий. = The validity period was short.
• Термін дії буде короткий. = The validity period will be short.

Because короткий is an adjective agreeing with термін.

Термін is:

• masculine
• singular
• nominative

So the adjective must match it:

• короткий = masculine singular nominative

If the noun changed, the adjective would change too:

• віза коротка = the visa is short-lived / short in duration
• правила короткі = the rules are short

Even though короткий comes after the noun here, it still agrees with термін.

It is a full sentence.

It has:

• a subject: термін дії
• a predicate adjective: короткий

The verb is is simply omitted, as is normal in the present tense.

Compare:

• Термін дії короткий. = The validity period is short.
  → full sentence

• короткий термін дії = a short validity period
  → just a noun phrase

That distinction is very useful.

Yes, but the result may sound different.

The most neutral order here is:

• Термін дії короткий.

If you say:

• Короткий термін дії

that is usually understood as a noun phrase meaning a short validity period, not as a neutral full sentence.

Ukrainian word order is flexible, but word order often affects:

• emphasis
• style
• whether something sounds like a statement or just a phrase

So for a plain statement, Термін дії короткий is the safest choice.

No. There are a few common alternatives.

For example:

• строк дії = period of validity
• термін чинності = term of validity, more formal
• період дії = period of operation/effect

Also, instead of короткий, you may hear more formal adjectives such as:

• нетривалий = not long, relatively short
• недовгий = not long

So depending on style, you might also see:

• Строк дії короткий.
• Термін чинності нетривалий.

Your original sentence is understandable and natural, especially in practical or administrative contexts.