tl;dr- This is called a capital loss.
You're said to realize the capital loss if you sell the Bitcoins at the lower price, such that you lost money due to having bought/sold them.
However, note that this isn't legal advice and I'm unsure about what the current legal statutes are surrounding Bitcoin.
Bitcoin's a capital asset (at least conceptually; dunno about legally):
A capital asset is defined to include property of any kind held by an assessee, whether connected with their business or profession or not connected with their business or profession.
It includes all kinds of property, movable or immovable, tangible or intangible, fixed or circulating.
Thus, land and building, plant and machinery, motorcar, furniture, jewellery, route permits, goodwill, tenancy rights, patents, trademarks, shares, debentures, securities, units, mutual funds, zero-coupon bonds etc. are capital assets.
-"Capital asset", Wikipedia [links omitted]
When a capital asset appreciates in value, it's called a capital gain, and may be subject to capital gains tax.
And when a capital asset depreciates in value, it's called a capital loss (and sometimes results in a reduced tax burden).
Until you actually sell the Bitcoins, the loss is considered unrealized:
What is an 'Unrealized Loss'
An unrealized loss is a loss that results from holding onto an asset after it has decreased in price, rather than selling it and realizing the loss.
An investor may prefer to let a loss go unrealized in the hope that the asset will eventually recover in price, thereby at least breaking even or posting a marginal profit.
For tax purposes, a loss needs to be realized before it can be used to offset capital gains.
-"Unrealized Loss", Investopedia [links omitted]
Once you do sell the Bitcoins, then you know how much money you've gained/loss. This is called realization:
DEFINITION of 'Realized Loss'
A loss is recognized when assets are sold for a price lower than the original purchase price.
Realized loss occurs when an asset which was purchased at a level referred to as cost or book value is then disbursed for a value below its book value.
The 2020 Bitcoin bubble is about to pop! Capitulation is coming & BTC price is about to drop hard!
Although the asset may have been held on the balance sheet at a fair value level below cost, the loss only becomes realized once the asset is off the books.
-"Realized Loss", Investopedia [links omitted]
If you buy Bitcoins and they depreciate in value, then:
You've suffered a capital loss.
Before you sell the Bitcoins, it's an unrealized capital loss.
After you sell the Bitcoins, it's a realized capital loss.