This is incredibly complicated and almost always misleading.
For example, in many countries the Catholic Church is called strictly the Roman-Catholic church. The higher up members of that community that acknowledges the papal supremacy call their own organisation most often just "the church". In that they are sharing this endonym with most other sects of course, Christian or not, in antiquity and today.
Then there are the early and still valid Notae ecclesiae:
The Four Marks of the Church, also known as the Attributes of the Church, is a term describing four distinctive adjectives — "One, Holy, Catholic and Apostolic" — of traditional Christian ecclesiology as expressed in the Nicene-Constantinopolitan Creed completed at the First Council of Constantinople in AD 381: "[I believe] in one, holy, catholic, and apostolic Church."
Or, in the Latin, then in Greek:
Et unam, sanctam, cathólicam et apostólicam Ecclésiam.
Εἰς μίαν, Ἁγίαν, Καθολικὴν καὶ Ἀποστολικὴν Ἐκκλησίαν.
[Quoted from:] "the 'Nicene Creed'. It is the only authoritative ecumenical statement of the Christian faith accepted by the Roman Catholic, Eastern Orthodox, Oriental Orthodox, Anglican, and the major Protestant denominations."
Then there is that dating problem. Choosing 1054 as the cutting point for the schism is totally unhelpful in that, except for having an answer in quizzes.
When did that schism begin?
Was it as early as 180 because of Quartodecimanism? Or as late as 1729, when Congregatio de Propaganda Fide actually forbid the community of sacraments.
There are a few options to consider as good names for "the church" before the 11th century:
- The completely correct: "church".
- The established: "Catholic church".
- The commonly understandable: "the universal church"
- The technically/historically descriptive: "Pentarchical church" or "Chalcedonian church" (as mentioned in comments by @NSNoob).
They all have advantages and disadvantages. Is the Chalcedonian church just the congregation in that locality? Is pentarchical just referring to the exact five sees? How ecumenical is the universal church?
Still other formulas like pre-schism Christian church etc. might be used.
It would very much depend on context, intended audience and readership which description to choose. It will be necessary to not just use "the word" (i.e. "a word") and be done with it.
There is no real solution for this as too many names have been proposed and used already. In any text you will likely have to define the way you are going to use that word to avoid misunderstandings. Without a given and strictly defined timeframe the words change their generally understood meaning. Without such prolegomena, confusion in a general audience is not just a possibility but will arise.
Bonus question to ponder about after having wrecked your brain on when the East-West schism started: Is it still in effect or did it end? If it ended, when did it end?
The ban and counter-bans from 1054 were abolished at the end the Second Vatican Council in 1965. But:
East and West since 1054
"Even after 1054 friendly relations between East and West continued. The two parts of Christendom were not yet conscious of a great gulf of separation between them. […] The dispute remained something of which ordinary Christians in East and West were largely unaware".
There was no single event that marked the breakdown. Rather, the two churches slid into and out of schism over a period of several centuries, punctuated with temporary reconciliations.
— Wikipedia East-West Schism
If you want to refer to the church or churches before the caesarian date of 1054 it would be probably easiest and most pragmatic to say "pre-1054 church(es)".