Clear my definitions

x = 5;
y = 7;
x + y

12

Clear[x, y]

Expand[(x + y) ^ 2]

x^2 + 2*x*y + y^2

Clear["Global`*"]

f[x_] := Table[i, {i, 0, x}]

f[{2, 4, 5, 6}]

Table[i, {i, 0, {2, 4, 5, 6}}]

SetAttributes[f, Listable]
f[{2, 4, 5, 6}]

{{0, 1, 2}, {0, 1, 2, 3, 4}, {0, 1, 2, 3, 4, 5}, {0, 1, 2, 3, 4, 5, 6}}

Clear[f]
 ? f

InformationData[<|ObjectType -> Symbol, Usage -> Global`f, Documentation -> None, OwnValues -> None, UpValues -> None, DownValues -> None, SubValues -> None, DefaultValues -> None, NValues -> None, FormatValues -> None, Options -> None, Attributes -> {Listable}, FullName -> Global`f|>]

ClearAll[f]
 ? f

Missing[UnknownSymbol, f]

f[x_] := Table[i, {i, 0, x}]
SetAttributes[f, Listable]
 ? f

InformationData[<|ObjectType -> Symbol, Usage -> Global`f, Documentation -> None, OwnValues -> None, UpValues -> None, DownValues -> Information`InformationValueForm[DownValues, f, {f[x_] :> Table[i, {i, 0, x}]}], SubValues -> None, DefaultValues -> None, NValues -> None, FormatValues -> None, Options -> None, Attributes -> {Listable}, FullName -> Global`f|>]

ClearAttributes[f, Listable]
 ? f

InformationData[<|ObjectType -> Symbol, Usage -> Global`f, Documentation -> None, OwnValues -> None, UpValues -> None, DownValues -> Information`InformationValueForm[DownValues, f, {f[x_] :> Table[i, {i, 0, x}]}], SubValues -> None, DefaultValues -> None, NValues -> None, FormatValues -> None, Options -> None, Attributes -> {}, FullName -> Global`f|>]

x = 5

5

x=.
 ? x

Missing[UnknownSymbol, x]

Remove[x]
 ? x

Missing[UnknownSymbol, x]

x
 ? x

Missing[UnknownSymbol, x]