Today one often hears the term on-premise in the IT industry (particularly outside the native English-speaking world). This usage is grammatically incorrect.
The correct term is on-premises.
“Premises”
The word premises refers to land and buildings. This word is always plural, and does not have a singular form without the –s suffix.
“The company premises were closed for the weekend.”
“We moved the application from Swiss Re premises to the Swisscom app cloud.”
“This table lists the pros and cons of cloud VMs and on-premises VMs.”
“Premise”
The word premise is a different word, and means proposition, thesis, or assertion. This word is most often used in philosophy, logic, and law.
“The premise that there exists an immaterial soul is not supported by empirical evidence.”
“The prosecution’s case relies on the premise that the defendant was on the train at the time of the theft.”
An alternative
The abbreviation on-prem is a neologism, but is concise and avoids the potential confusion.
A note about hyphenation
Adjectival phrases — phrases that modify a noun — should be hyphenated.
Examples
“Pension contributions are deducted from your before-tax income.”
“Gastronomy services are provided via a pay-as-you-go approach.”
“Remember to configure Outlook to send out-of-office notifications.”
“We observed higher performance with the on-premises database.”
“Following the migration, we decommissioned our on-prem servers.”
