Sequoia or Safari?

Sequoia is one of Apple's many macOS versions.

You need to have a 2019 iMac to run Sequoia.

Identify your iMac model - Apple Support

Safari is Apple's default web browser.

Safari - Official Apple Support

The latest macOS that a 2017 iMac can run is Ventura 13.7.2.

And as you've been told Safair is not a macOS it's a web browser.

Wayne511 wrote:

iMac 27″, macOS 15.8

The only iMac from 2017 that can run macOS 15 Sequoia is the high-end iMac Pro. Is that what you have? Black case?

For your 27" 2017 iMac, macOS 13.* (Ventura) is the end of the line. In recent times, each major version of Safari has supported the "most recent three" versions of macOS.

Thus, the latest major version of Safari for Ventura is Safari 18 – two major versions behind Safari 27 – the version which runs on Sequoia, Tahoe, and Golden Gate. The latest release of Safari 18 seems to be Safari 18.6, released over a year ago on July 29, 2025.

Safari Release Notes | Apple Developer Documentation

At some point, you may want to install a second Web browser, such as Firefox, whose current version still supports versions of macOS as old as Catalina.

To add …

Some confusion may also exist as updated versions of the Safari browser are usually packaged with updated versions of the MacOS.

I suspect that many users are unaware when they’re installing Safari nn.nn.n… they just know that they’re installing an update (or upgrade) to their MacOS.

Addendum: If you're considering getting a new Mac consider the following:  a 12‑core CPU, 12‑core GPU Dual 16‑core Neural Engine Mac Mini M6 with 16 GB RAM and 1 TB SSD ($1399 US) with a 32" LG 4k monitor ($331 US) from Amazon.com)  is $1716 which is $480 less than a similarly configured new 24" M4 iMac ($2199 US). Keep your keyboard and mouse from your current Mac for use with the Mini. If you recycle don't include the mouse an keyboard.

I got the previous Mini M4 with a 32" LG 4K monitor w/speakers for the same price as a 27" monitor (didn't know the Mini had a speaker).  There are many monitors available in 27" size for about $100 US to $500 US depending on what features you want.  I love the extra screen real estate of the 32" monitor.

I had an 2017 i7 iMac with an SSD boot drive and a new M4 Mini runs circles around it.  The new M6 will be even faster.   

You can use the keyboard and mouse from your current iMac. Don't include them with the iMac if you use Apple Trade In or recycle with the  iMac.

Just some food for thought.

If you are looking at your upgrade options in System Settings then Sequoia is the OS update available but, where it can, Apple keeps Safari more up-to-date (in terms of version number) than OS.

You're welcome.