As mentioned, what you are referring to are called hardware wallets. They are small devices, a bit bigger than a USB drive, which will store your keys offline and require a computer or mobile phone to use them.

Ledger and Trezor are by far the two biggest ones, although there are others available such as ColdCard and KeepKey.

My recommendation would be for Ledger. There has been a major flaw revealed about Trezor devices recently which would allow a knowledgeable attacker to extract your seed, which makes them currently a worse choice than Ledger devices.

You can get a Ledger Nano S for around 60 bucks. Their newest model, the Nano X, costs more but comes with Bluetooth functionality and larger storage, which is important if you own an iPhone (Android phones will work with the Nano S via a cable, whilst the iPhone will only work with the Nano X via bluetooth) or if you are interested in holding many different altcoins.

Ledger usually put on a Black Friday sale, so wait a few weeks before buying and you'll hopefully get a nice discount. Always make sure you buy directly from their website or their official Amazon store and not from a third party reseller, for your own safety.

Wow. I've got some really interesting replies from this thread. Thanks to all the contributors so far. I have a client who's interested in developing a cryptocurrency, I'll get this info across to him ASAP.

I think most cryptocurrency wallets are portable because most of them can be access through mobile phones and laptops. But if you are looking for a offline wallet, you better used ledger or trezor because both of them is a good quality hardware wallet and it can be trusted, so it is safe to store all of your funds. You just be careful in bringing your hardware wallet wherever you go because it will be difficult for you to recover all of your funds if you lost it.

The Coldkey you mentioned is a physical cold storage wallet and is only in FORM of card. I wrote it just to clarify that someone would not think that can use it as a normal card.   And it is more intended for collectors and enthusiasts than for ordinary users. When it comes to its security, we will be able to check it when it will be created. Anyway, this is a very interesting project.

Portable wallet hardware that can be taken anywhere is sold in the online market. The most popular are Ledger Nano S and Trezor. But there is another one that tells me it's interesting, which is imKey, which was developed by imToken. imKey has a portable form and is integrated with the imToken mobile app. to be able to buy it you can directly visit each other's website or you can try searching for it in the Online Marketplace.

Hardware walletnis secured to stored or couns investment for long term hold yes it's profitable if the coin stored in the wallet are pumping, you can use MEW or nano ledger, but if looking profits I think you are not looking wallet to make profits. If you looking how to make profit you are looking best excxchange because trading is the best to get earn profits.

Portable wallet or hardware wallet is the safest way to store your funds, especially if it is huge. I have a ledger wallet which I bought from a friend. Just be cautious on buying it online because there are instances that some sellers are opening it and getting the seed before selling them. It is worth investing if it is or your security.

That's not accurate. Hardware wallets a perfectly able to be used for "short term or trading purposes", and moving coins back and forth an exchange. Mobile wallets should only be used for very small amounts of day-to-day spending bitcoin, which you would be willing to lose, if you don't want to also carry a hardware wallet around with you.

Using a hardware wallet given to you by a third party, even if it is a third party you trust such as a friend, is far more risky. Even if you trust your friend not to have tampered with it, do you also trust them to have kept it completely secure the entire time they owned it and not have exposed it to anyone else who could have tampered with it. What about where they got it from? Do you also trust them to have bought it directly from the source. If you are buying a hardware wallet, you should buy only directly from the official site, and never from any other retailer, third party, friend, or so forth.

No. If you are going to be storing your life saving's worth of bitcoin in a device, why would you want that device to have been passed around and handled by who knows how many people in the supply chain and third party store? You are exposing yourself to a huge amount of unnecessary risk. Only buy direct from source: https://shop.ledger.com/

Some mobile companies are genuinely thinking about pre-installing a Cryptocurrency wallet that would be something that people could use and it would be secure too ,

 at the same time you should understand that you can opt for the hardware wallets which are made by various companies and the most trustable one is the Ledger which is primarily a French startup .