The meagre subsets of a fixed space form a σ-ideal of subsets; that is, any subset of a meagre set is meagre, and the union of countably many meagre sets is meagre.
General topologists use the term Baire space to refer to a broad class of topological spaces on which the notion of meagre set is not trivial (in particular, the entire space is not meagre).
Descriptive set theorists mostly study meagre sets as subsets of the real numbers, or more generally any Polish space, and reserve the term Baire space for one particular Polish space.
The complement of a meagre set is a comeagre set or residual set.
A set that is not meagre is called nonmeagre and is said to be of the second category.
Note that the notions of a comeagre set and a nonmeagre set are not equivalent.
A subset of a topological space is said to be meagre in a meagre subset of and of the first category in if it is a countable union of nowhere dense subsets of
A subset that is not of first category in is said to be nonmeagre in a nonmeagre subset of and of the second category in 
A topological space is said to be meagre (respectively, nonmeagre) if it is a meagre (respectively, nonmeagre) subset of itself.
It may be important to distinguish nonmeagre subspaces from nonmeagre subsets. If is a subset of then being a "meagre subspace" of means that when is endowed with the subspace topology (induced on it by ) then is a meagre topological space (that is, is a meagre subset of ). In contrast, being a "meagre subset" of means that is equal to a countable union of nowhere dense subsets of The same warning applies to nonmeagre subsets versus nonmeagre subspaces. More details on how to tell these notions apart (and why the slight difference in these terms is reasonable) are given in this footnote.[note 1]
For example, if is the set of all positive integers then is a meager subset of but not a meager subspace of If is not an isolated point of a T1 space (meaning that is not an open subset of ) then is a meager subspace of but not a meager subset of 
A subset is called a residual subset of and is said to be comeagre in if its complement is meagre in . (This use of the prefix "co" is consistent with its use in other terms such as "cofinite".)
A subset is comeagre in if and only if it is equal to an intersection of countably many sets, each of whose topological interior is a dense subset of
Importantly, being of the second category is not the same as being comeagre — a set may be neither meagre nor comeagre (in this case it will be of second category).
The terms first category and second category were the original ones used by René Baire in his thesis. The meagre terminology was introduced by Bourbaki.
Any subset of a meagre set is a meagre set, as is the union of countably many meagre sets.
If is a homeomorphism then a subset is meagre if and only if is meagre.
Every nowhere dense subset is a meagre set. Consequently, any closed subset of whose interior in is empty is of the first category of (that is, it is a meager subset of ). Thus a closed subset of that is of the second category in must have non-empty interior in 
The empty set is always a closed nowhere dense (and thus meagre) subset of every topological space and it is also a meagre subspace.
A singleton subset is always a nonmeagre subspace of (that is, it is a nonmeagre topological space). If is an isolated point of (meaning that is an open subset) then is also a nonmeagre subset of ; the converse holds if is a T1 space.
The set is a meagre subset of even though is a nonmeagre subspace (that is, is not a meagre topological space).
A countable Hausdorff space without isolated points is meagre, whereas any topological space that contains an isolated point is nonmeagre.
Because the rational numbers are countable, they are meagre as a subset of the reals and as a space—that is, they do not form a Baire space.
A meagre set need not have measure zero.
There exist nowhere dense subsets (which are thus meagre subsets) that have positive Lebesgue measure.
Relation to Borel hierarchyEdit
Just as a nowhere dense subset need not be closed, but is always contained in a closed nowhere dense subset (viz, its closure), a meagre set need not be an set (countable union of closed sets), but is always contained in an set made from nowhere dense sets (by taking the closure of each set).
Dually, just as the complement of a nowhere dense set need not be open, but has a dense interior (contains a dense open set), a comeagre set need not be a set (countable intersection of open sets), but contains a dense set formed from dense open sets.
Meagre sets have a useful alternative characterization in terms of the Banach–Mazur game.
Let be a topological space, be a family of subsets of that have nonempty interiors such that every nonempty open set has a subset belonging to and be any subset of
Then there is a Banach–Mazur game
In the Banach–Mazur game, two players, and alternately choose successively smaller elements of to produce a sequence
Player wins if the intersection of this sequence contains a point in ; otherwise, player wins.
Theorem — For any meeting the above criteria, player has a winning strategy if and only if is meagre.
Baire category theorem – On topological spaces where the intersection of countably many dense open sets is dense
^This distinction between "subspace" and "subset" is a consequence of the fact that in general topology, the word "space" means "topological space", which is a pair consisting of a set and topology, and (similarly) the word "subspace" means "topological subspace"; consequently, "subspace of " refers to the pair consisting of the subset together with the subspace topology that it inherits from whereas "subset of " refers only to the set. Consequently, if the subset lacks any topology then " is meagre of subset of " is not well-defined, leaving " is a meagre subset of " as the only possible meaning of " is meagre". But if is endowed with a topology then (by definition) " is meagre" means " is a meagre subset of " Saying " is a meagre subspace of " is just a combination of the following two statements: (1) " is a subspace of ", which by definition means that is endowed with a topology that is equal to the subspace topology induced by on it by (denote this topology by ), and (2) " is a meagre space", which by definition means " is a meagre subset of ". However, if happens to be endowed with a topology (say ) then the statement " is a meagre subset of " does not mean " is a meagre subset of " because in this statement, is being considered as a set (and not as a topological space). The same is true of a statement such as "let be a subspace of that is a meagre subset of " and its more succinct equivalent "let be a subspace that is meagre in " (note that the meaning is completely changed without the words "in ").