A trend can be either negative (decreasing) or positive (increasing).
For that matter, a trend can be neutral (flat).
A trend is simply the continuation of what has gone before.
The Longman on-line dictionary defines it as follows: a general tendency in the way a situation is changing or developing
So your sentences could be:
Sales declined to 10 in November, and to 5 in December.
This trend repeated the following year.
As you have already mentioned that sales declined in November and December, it is not necessary to state, 'This negative trend repeated the following year', although doing so would not be frowned on.
1/
If you are talking about 'sales' generally, there is no need to precede it with the definite article.
If you are talking about a specific 'sales of something', then you may precede that with a definite article, although it is not essential, e.g. (The) sales of automotive parts declined in August.
2/
Because you are talking about two different trends, the results of sales in November and December in two consecutive years, it is correct to say, 'The trend repeated...'.
If you had said, 'This trend repeated the following month', this would have been wrong, because the following month would have been a continuation of the trend in the two previous months.
In this case you would have had to say, 'This trend continued the following month'.
ell.stackexchange.com/users/76234/James
You can use the word decline as a noun to mean a negative/downward trend.
decline has many synonyms , but most are specific to particular circumstances.
fall would work in this context.
ell.stackexchange.com/users/30978/JavaLatte
In a more technical vein, you could talk about a positive slope (increase) and a negative slope (decrease) but as you mention sales as your example this may not be as common - if the example was voltage change over time then it wold be fine.
ell.stackexchange.com/users/73674/Solar Mike
Another word for declining trend is downturn .
ell.stackexchange.com/users/10264/Tᴚoɯɐuo
One thing that immediately came to my mind is pattern :
The sales declined to 10 in November, and to 5 in December.
This pattern repeated the following year.
It's a little more general that trend , because it can describe something other than a steady incline or decline, but it's still a way of describing the behaviour of something.
From Merriam-Webster : 7 : a reliable sample of traits, acts, tendencies, or other observable characteristics of a person, group, or institution • a behavior pattern • spending patterns •
the prevailing pattern of speech
It does not mean negative on its own, but you can easily add that adjective and describe a negative pattern which will be understood in context.
Alternatively, if you really do want a single word that's specifically negative, try erosion :
The sales declined to 10 in November, and to 5 in December.
This erosion repeated the following year.
The word has the benefit of having a negative emotional connotation to it in addition to just a metric.
From Merriam-Webster, erosion is "the action or process of eroding," which redirects to erode : 1 : to diminish or destroy by degrees: c : to cause to deteriorate or disappear as if by eating or wearing away • inflation eroding buying power
ell.stackexchange.com/users/73781/Jason Bassford
Is there a word for that?  Example sentence:   The sales declined to 10 in November, and to 5 in December. This __ repeated the following year.   Note: I checked the synonyms for  trend . But none of them imply a negative/downward trend.
Is there a word for "negative/declining trend"?
ell.stackexchange.com
<word-request>
ell.stackexchange.com/questions/174479
ell.stackexchange.com/users/43585/alex
One might say negative or downward trend, or decline/downturn. A trend can increase or decrease, and is the continuation of the past direction. Decline or downturn are two nouns that could be used.
A trend can increase or decrease, and is the continuation of the past direction. Decline or downturn are two nouns that could be used.